Tempremental 2strokes! Crankcase and top end compression is important on 2strokes. It could be that you are lacking a little of either or both.
Check head bolts are tight with no signs of blowing, cylinder base to crankcase ditto, and carb. flange joint is good.
You seem to have checked everything else (including fuel mix?)
Difficult (or non starting ) when hot can be caused by loss of compression due to wear on piston rings. Replacing rings, if necessary, should not be difficult if bore is undamaged. De-glase the bore though, or the new rings will never bed in.
Mr. Toad
July 2008