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Troybilt Pony (circa 2008) was working fine but started to have problems moving forward. The symptoms are very similar to this report: https://www.lawnmowerforum.com/threads/troy-bilt-pony-stops-moving.60749/ ,however after going through those solutions, the problem still exists. My primary question at this time is this: "How much shoul the 'drive tensioner pulley arm' move?" and "Would its lack of movement cause these problems?".
Here are the symptoms that are different from the above referenced report...

The mower runs great but after about a minute or two starts to slow down, "skipping", and then stops moving forward. This can be while mowing or not. If I release the pedal and press again, it may "take off" again. However sometimes that fails. Shifting into reverse, moving a bit, them back to forward, and it goes fine, but just for a short time. Note that it will move fine in reverse 'forever" with no problem.
The belts are fine - not new but good shape, and the pulley turns. However I noticed is that the "drive tensioner pulley arm" does not move. Since it has a spring attached it seems logical that it should. The spring is rusty but still in good workable shape.

So is the problem the tensioner arm or something else? The ability to reverse leads me to think that it may be a transmission problem.

Thanks for any help.

When was the last time you changed the drive belts? If it’s been three plus years I would start there. The tensioner arm should move enough to put constant tension on the belt.

Thanks @Rivets!
I never changed the belts as I just bought it.
I'll tear it apart and see what's got the tensioner arm frozen stuck.
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I have changed the two drive belts, long and short. The belts are tight and pulleys turn freely. I did loosen and lightly lube the joint where the tensioner arm and the variable pulley arm are attached to the frame. This allowed the tenioner arm to move more freely. The only part that seems a bit worn/week is the spring for the tensioner arm, but it still keeps tension.
All this and the same problem occurs; goes fine in reverse but will stop moving completely on level ground in forward. There is often a clank sound when I depress the clutch/brake after it stops in forward. Many times after that it will move forward again for a bit. However the only way to really get it to move forward again is to shift to reverse, move any distance, then back to forward.

Anybody have ideas on this problem?

Did you check to see if the variable speed pulley is moving freely up and down when you had the belts off?

Yes, the pulley can be lifted/pushed-up easily and drops easily on its own. In fact it was a pain to keep it up and out of the way while installing the new long drive belt over the "belt keeper wings" and into the bottom of that pulley.

Just ran the mower around a bit. Still the same symptoms although I noticed a couple things:
1) seems to work fine when 'cold' but eventually fails consistently.
2) checking the pulleys and belts after running a while in forward, I found the variable speed pulley surfaces hot, as though the belts are just sliding across it.
3) also the variable speed pulley is not movable by hand except to grab and pull the belt. It moves with the belt.
4) then running in reverse a bit, there seemed to be no difference in that pulley.
5) the movable portion is in the down position and cannot be moved by hand.

I'm going to do more testing but have a couple questions:
Can the variable speed pulley be lubricated? Is that worth trying?
Is there other tests that I could do to determine the problem?

THANKS!

If the mower will go FULL SPEED backwards but not full speed forwards then the problem is in the diff
The lever that has the F-N-R shift on it slides a dog left <-> right to change the direction the wheels move
Usually when that wears you get a rapid clicking like a movie gattleing gun sound .
Download the IPL for your mower and look at the system
Some time the gear teeth wear but usually it is the forward side bush that wears
Replace the bush ( doing both won't hurt ) and replace the lubricant which is generally grease that has gone old & dried out
The shift arm & connecting levers also wear so look at the plunger on the end of the shift lever.
IF it does not move the same distance Left <=> Right then that may require attention
MEasure the position of the plunger then go mow
When it stops moving check it again
If it has moved then the detent may be faulty
It is only a ball bearing & spring .
Fairly common to find the spring rusty or broken , again a cheap fix just messy & time consuming .
There isa funny looking spring on the shift lever under the rear guard
These fall off and that allows the shift rod to move at the gearbox end while appearing to be in the right spot on top of the mudguard .
A proper bell crank and pivot is what should have been used rather than a twisted & bent shift rod but the latter is a lot cheaper .

@bertsmobile1 Thank You!! let me clarify what you are saying
IPL ==> exploded parts diagram?? or some such.
So I take it that you are saying the problem is likely the shifter mechanism not staying engaged properly. I'll take a good look and give it a test. Would be nice if a new spring fixes it!

bTW My mower does not make the "gattling gun" sound but it does click occasionally when shifting to reverse or depressing the clutch/brake when it stops moving. However it's more of a "clunk".

Appreciate the help!

With the engine off, have someone move the pedal and the selector in a variety of positions and see what you can see. With the pedal depressed, see if there is any up/down movement in the center sheave, it may be a bit "sticky".

Illustrated Parts List but you seem to have the meaning.
Yes
To the belt system there is no difference between going forewards or backwards except for the torque reaction which allows the whole transmission to tilt foreward or backwards if the torque straps are loose, broken of missing in which case the belt flips off .
The fact that your mower looses motion quietly suggests a belt
The fact that reverse is fine & forward is a problem suggests the belts
So what I really would like you to do is try to mow your lawn entirely in reverse.
If after a reasonable time going backwards the problem reappears then that again points to belts not the axels

The entire drive system is a tug-O-War between the belts.
The top belt tension is set by the spring which does get weaker over time and will cut grooves in mounting points so provide less tension.
This makes the mower go faster in lower speed settings.
SO the speed is changed by the tension in the front belt and this is done 2 different ways.
If your mower has "X" number of speeds then those are done by preventing the clutch / brake pedal fully returning thus reducing the tension that the front spring(s) can apply to the front belt
This type suffers greatly from the springs going weak , breaking, falling off or chopping out their mounting points and in this case the mower goes slower to the point of stopping altogether
The size shape strength & number of springs varies from model to model as does the mounting points.
Also if you have removed the springs before the problem happened then good chance they did not go back in the correct position
The other variation the front belt tension is applied by your foot so you effectivly have an accelerator and the springs just return the pedal to neutral .
Without the actual model & serial numbers from the tag under your seat we will not know which system you have and what spring set up you have
Some springs mount to tabs. others slots or bolts or just holes in the frame