Check the head like '01KX said. You might have over torqued the head. Are the bolts coming loose? You might consider lock-tighting them. I was having the same issue and it was the bolts coming loose. I ended up having to send my head off because I warped it pretty bad. I had EGorr do my KX500 twice, head, porting, boring, ect. My Cr125's (both 144 kits), and my CR250 (260cc). So far has not screwed on me yet, just alot of wait. But it was worth it.
Check your heads alignment. Also use a different gasket from Moose. I use the super thick gasket with huge success. Try cosmetic, best stuff in my oppinion.