could be head gasket, (not that I wish that on anyone) but having worked on several overheating subarus I know how fickle the gaskets can be. it might just take a couple G's to get the air bubble to fall into a place where it prevents flow. Nice thing is the heads are short and don't often warp.. although the single overhead cam engines tend to be better.

check for telltale signs air bubbles in the coolant, or disappearing coolant, white smoke, they don't often milk up the oil or vice versa. it's usually between the water jacket and cylinder. but without pressurizing the cooling system it's going to be hard to diagnose.

I've seen 2.5, 2.0, and 2.2 motors all shit the gaskets.. and more often then not the overheating is intermittent. I thought the STI semi closed deck was supposed to be better, but who knows.