If your valves are not holding the lash you may have to do the whole top end work and I would go ahead and do the piston and ring set.....do not just replace the rings get a new piston too! I would just re-adjust the valves and try it again but I'm not you, let us know what you find out.
Also if the carb is getting really hot it will act the same way as you've said. Mine was doing this so I made a heat deflector and installed it under the carb to keep the radiant heat from the carb and it fixed mine problem.
I think the gas is trying to boil off in the carb bowl.

 

does anyone know of a way to adjust the carb for lean/rich while its on the quad? there is a screw on the front close to the head but i cant figure a way to get to it please any help would be appreciated also looking for ideas or pics of a winch mount for this,i will make another post about the winch mount