If you drained the coolant make sure you bleed the air out of the head. There's a small bolt and crush washer on the right side that you loosen to do this. Have you verified if the fuel tank is venting correctly? When it dies, pull the spark plug and make sure you still have spark. There have been a few XC's here that have had pickup coils go bad once they get hot. Another common thing is for the valves to get tight and cause issues. If they are tight on clearance they will loose clearance once hot, start holding valves open, and be hard to restart until the engine cools off. Verify they are within spec and adjust as needed. The intake boot between the head and carb on these is prone to cracking and sucking air. Sometimes you have to have this boot all the way off to see the cracking from the inside.