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I’m trying to fix up some old equipment and am currently working on an MTD Yard Machine lawn tractor, model 13AK608G062, from 2000 I believe, with the single speed transmission. It needed a new lower belt, so I replaced that with an OEM belt. It moves now, but not smoothly. I may go along nicely for a few seconds, then the the machine starts to move in spurts, starting and stopping. The variable drive pulley is in good shape, smooth and turning easily. The engine runs nice and smooth. The drive pedal adjustment is okay—the tractor does not move when the pedal is released. There doesn’t seem to be anything else to check with the belt. Could this be a transmission problem? If so, what should I be looking for?

diagram shows what appears to be 2 drive belts, like some MTD's have, the main long one and shorter one..

Two things. the two drive belts have to be replaced at the same time or it will cause drive issues. There could also be an issue with the shift collar and the gear that they interface with. If the notched surfaces get worn it will cause jumping in and out of gear.

A 21 years old I can safely say that the pivot holes on the rear belt tensioner are and the front belt tensioner arm are flogged out oval .
This causes erratic movement of the tension arm as some times they rotate and some times they slide in the flogged out holes and at no time is either pulley in the right position .
I like varidrives because they are near bullet proof and the cost of repair / replacement is reasonable comparred to the megga cost of rebuilding hydrs.
However they do need to be serviced, those pivot points need some lube on a regular basis and if not replacement or repair.
Down here mower parts are all handled by the space shuttle company ( by the price they charge ) so I build up the worn away areas then recut the holes because 2 hours of my fee is always less than the cost of the parts alone.

Thanks for the suggestions. If it rains tomorrow I'll do some checking; we've had 20+ days of rain this month here in central PA, so if it's not raining I'll be busy with other chores. We use the mower as a tow vehicle now, not as a mower, so I can let the work sit if necessary. But I really would like to get it running again because we use grass clippings to mulch the garden, and collecting the clippings with a lawn sweeper is a lot easier than using a rake. I replaced just the lower belt because the top belt looked almost new, definitely not as worn as the lower belt. But I'll get a new top belt and try that.

In the mean time, mow your grass in a clockwise fashion.
That will throw all of the grass to the right and eventually a heap in the middle of the lawn.
If it is big then several circles
It is what we used to do before mowers had catchers .
Mow a small block in the middle then go out to the edge and mow into the middle patch.
Occasionally you had to rake into the middle a few times if there were a lot of clippings that were overloading the deck .

Rain is imminent, so I worked on the mower. A new top belt didn't make any difference. The pivot holes are in good shape. I got the mower used and I know its history. Even though it is 20 years old it did not get used hard. In fact, I got it because it had sat unused for several years. Now I'm assuming that the problem is in the transmission. I drove it this morning without ear muffs and it certainly sounds as though there are some strange noises coming from the transmission. Removing the transmission is not a summer time project, so I'll put the this mower into the barn for now. My wife would really like me to get the old Cub Cadet push mower running again, so I guess I'll start to work on that mower.