Im not ruling out headgaskets, but of course Id certainly like it to be something else. I replaced my IAC 3 days ago. The problem is, after like 15-20 minutes of driving, the needle is past the M on the gauge. It NEVER used to do this. When I first got it (1 month ago) it would barely approach the N. Just now, I burped it and filled it with some antifreeze and such. Drove it around, it got hot. When I parked and opened the hood, the coolant in the res was bubbling. I dunno if it was boiling, but it caused me to dump a good amount of coolant on the ground.