Glad to hear you solved it without a rebuild!
Originally Posted by laxton
I HAD THE EXACT SAME PROBLEM! Changed coil packs and plugs and cleaned mafs.. Still nothing. Done a comp test and it was 80psi! Put in a scope and seen bad valves so I got all new ones. When the head was removed I descovered I has multiple cracks in my head. (explanes why it runs fine when cold) its bc the cracks were small but as it heated up the cracks would expand and I would lose more compression. Please take my advice and get a test ran to save u more time.
Sorry to drift off topic, but since metal expands under heat wouldn't it close the cracks as it got hotter?
Not to mention, small cracks in the heads of a WRX are fairly common.
I'd say your valves were the only issue.