Restoring a 2001 Polaris Sportsman that had been in a garage, unused for a few years.
New oil injection pump, oil lines, cleaned everything out of the oil tank and fuel tank.
Replaced all the fuel/vent lines, stator, coil, plug, carb, plastic cylinder shroud and rubber air hose.
First attempt it cranked right up and after a bit settled into a good idle.
Drained the transmission and flushed it with Marvel Mystery Oil and Power Steering Fluid , then drained that and filled it with 0-40.

Problem is after about 10 minutes of riding, the bike locks up and dies and the starter won't turn the motor.
If I roll it back you can feel the gearbox/clutch/belt release, and then it will start and run fine again, until you get to the right speed or whatever it is and it locks up again.
I've ordered a belt, it was really stretched. I don't really see anything wrong with the clutch or chain.
Any suggestions?