andy, welcome. Most folks on this forum have a 2nd gen Equinox or a Terrain so you may not get too many answers. My long time experience with engines would suggest your on the right track when you suggest a problem the head/head gasket. The only way you'll really get the answer is to have the engine top end disassembled to exactly define the problem. It's impossible to trouble shoot this without digging into the engine. Good luck, Ron