1998 Subaru Outback overheating

In the past mionth, my 1998 Subaru Outback has been overheating, when the car is driven when warm on local roads or highway, the heat guage would peg to the maximum, We tried lowering the heat, that didn't work, one-time, the car overheated so much, that a pressure valve in the radiator blew out and the car had to be flat-bedded to the michanic, another time, the heat guage went to norrnal when the heat guage was moved to the cool (right direction). The thermostat was replaced twice, then the water pump, car still overheating. Recently during overhating, the radiator (after the pressure valve was replaced) wouldn't take a radiator fuild when ideling. My mechanic thinks the problem might be in the Head, or the gasket. The car has the same power as before and there is no smell of burning fluid from the engine. Any ideas? Thanks, Joe