Ok - here's an update. Ran it down to 1/8 of a tank last week and drained the rest out. Filled back up halfway with fresh gas. Ran okay for a bit after that. Started it up today and cut the flat part of my yard (about an acre). Ran fine for about 45 minutes. I turned off the mower deck to move to a different part - still all flat - and after driving about 30 yards I turned the deck back on. It immediately started struggling. If I tried to move forward (still completely flat ground) it would bog down and almost die. I had to shut it off completely and then restart it. After restarting, it ran fine again. Grass is finish cut so the bogging down had nothing to do with grass height.
Came in for lunch and then went back out to cut around my house. Immediately started struggling again. Starting and stopping made no difference.
No clue at this point, so probably headed to the shop for the second time in a little over a year that I have owned it....