Yup already suspected this when Superfly first posted his problem. Please see:
http://www.perth-wrx.com/vb/mechanic...ownpipe-2.html

Snap-on gas tester has been performed and the liquid changes color. Also did a pressure test on the cooling system and there are no coolant leaks externally.

To begin with, the cause of the simple cooling system failure was a leaking radiator cap. Changed the cap and bled the cooling system, but there were still consistent air bubbles coming out of the reservoir.

Although it is rare for the headgasket to be damaged on a wrx, prolonged over-heating can cause damage to the heads themselves too. Dont foget also the the block is aluminium too. Recently had a Honda S2000 customer who overheated his engine. He not only damaged the headgaskets and head, the block itself got so hot that the piston walls were deformed.

Regards,
Mao/ Evo R