KX85 Help...I'm confused

I am so confused....a friend gave us a kx85 to do some flat track racing and we are having some serious problems with it. When my son went out for pracitce it ran fine for about 7 laps(indoor short track). Sat in line waited to go again made it 4 laps and the bike locked up. He was going down the back stretch and it locked up the rear tire and turned him sideways. I thought for sure he siezed it. I pushed it off the track and went to kick it over and it fired right up. He went again and made it 3 laps and it did the same thing. I thought that it may have been vapor locking so I checked all his fuel lines and gas cap. Everything looked and operated fine. I cleaned the carb and messed with the jetting with no improvements.

I thought that it may not be getting enough fuel but not getting enough fuel wouldn't make it die and lock everything up. My son has siezed a motor before and he said that it felt like that.

I can't figure out why it would lock up and then kick over. I was wondering if I was having a cooling problem and the cylinder was heating up enough to stop the piston from completing the firing sequence, which in turn stopped the bike dead in its tracks.
I am getting ready to tear down the motor but I was wondering if anybody has any idea what this could be. This bike has been sitting for awhile so I don't know much about it right now but It appears that I am going to have to get very familiar with it.

Thanks

Never thought about the tranny....That would make sense but There were a couple of time while he was waiting to practice it would just die....

I will check the oil just to make sure there are no other issues.