Overheating

It sounds like a head gasket failure. Combustion gases from cylinder are leaking around the head gasket and pressurizing the coolant system. Head gasket failures can leak oil or water external to the head (a visible external leak), or they can leak combustion gases into the coolant system (your symptom) or they can leak coolant into the oil. Just depends on where the head gasket fails. Head gasket failures are common on the 2.5L Subarus, especially prior to 2010 when the gaskets were improved. You probably only have one failure, but I would replace both head gaskets if doing this repair. Depending on how much the engine has overheated (and multiple times), the cylinder heads may have warped some. It would be a good idea to have them checked by a machine shop (and/or resurfaced) when you replace the head gaskets. Don't wait too long, this overheating will get worse and happen quicker over time as the head gasket deteriorates more.