Im hardpressed to think its ur head gasket at this point..is ur car overheating? My first thought is the timing..ur exhaust valves are opening during part of the combustion cycle. Do u have an adj fuel reg? May want to play a bit with that also. When was ur last tune up? If ur stuck on the head gasket thing, which I hope ur not, may want to do a compression test on ur cylinders....usually a good test to detect any kind of leaking.