I'd get the head checked out. Either warped or there's a crack in a combustion chamber. Not everyone agrees with me on this one which is a fair call but as a rule of thumb I'd never get a head skimmed if it is out of spec. Especially the 4G64 heads. They can be temperamental and seem to be more prone to warping than other Mitsubishi heads - to the point where cams have been snapped from binding up. If it's just a bad head gasket from new (it does happen) or you had installation issues, you should see a spot on the deck of the block or the tops of piston/inside of combustion chambers where coolant has scoured the surface clean. If it's as bad as you say it'll be pretty obvious.