Just recently, I thought I needed a good radiator flush. Took the 92 si that I have to the radiator guy up the street. Yoshi. My symptoms were low coolant, and a SHOOT in temp getting off the freeway when for 30mins of 65mph, everything was FINE! I'd pull over, wait, fill with water, and was good for city for another 20 min. then it would happen all over again. Thought it was my thermoswitch. First thing Yoshi does is take off my oil cap. Turns it over and wipes is finger on the inside top. It's not oily, it's WET!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Like coolant wet. "Head gasket." He asks me if the car is warmed up, I tell him "NO, cause I actually live 3 blocks from you." He squeezes the top radiator hose and it feel like a balloon. He tells me that the pressure building up in the hose is due to the head gasket being blown, in addition to the coolant in where nothing but OIL should be. Eventually the coolant is going to rust out the internals.

Hope it helps.