My experience with a seize is either an abrupt stop of the piston or a tightening up of the motor. If you know someone with a 2 stroke pressure tester, it would be worth the time. Are you getting any extra spooge out of the exhaust or at the headpipe? After it cuts of what do you have to do to get it running again? You could try this, if you haven't already, take a new plug with you and replace it when the bike shuts off. If it starts right up you know something is causing your plug to foul. If it doesn't start right up then the whole crank seal leak may be the wrong way to go. It really depends on the restart circumstances.
1993 XR650L; 14T CS, Dave's carb mod, Modified stk muffler, Airbox Mod, Battery relocation, Bar risers, Uni filter, No passenger pegs(to make it super lite), Blue powerband.