MTD runs then quits going forward/backward while the engine is still running

18 Yard-Man/MTD
42"
16 hp, original engine
About 500 hours on it

Yesterday finished mowing about 2/3 of an acre, during the mowing the following happened a couple of times.
1) mowing along in 4th gear
2) forward motion slows from normal to 50% over the space of about 3 seconds but ENGINE IS RUNNING *AT THE SAME SPEED*
3) this last for 2 or 3 seconds, short enough that I'm not certain it is happening
4) then forward motion resumes at normal speed

I finish mowing then park it, then do a little testing and the following happens.
1) moves forward just fine for about 5 seconds after starting, then slows, then stops but ENGINE IS STILL RUNNING AT THE SAME SPEED
2) does *exactly* the same thing in reverse
3) when I switch from forward to reverse (after it has stopped moving), or reverse to forward (after it has stopped moving), it runs in the appropriate direction for maybe 3-5 seconds, then slows down but the engine is still running at the same speed

My guess is belt related but I don't smell any burning or heat, no smoke from burning grass underneath. Note belt is only 1 season old, maybe 25 hours.

Mower is pretty worn out so I'm trying to make the decision to kill it or not.

Suggestions?

Many thanks,
Dan