[goodhart]

What kind of headgasket have you been running? I have used the Cometic MLS with ARP's and have had the same issue. I am pretty sure what has/is happened to mine is that the head lifts under a decent amount of boost ( I run 23psi ) and combustion pressure seeps into coolant passages causing the car to push coolant. Even though the head gasket didn't get damaged, it unsealed during boost causing the pushing/overheating problem.

So, if You are using a Cometic MLS or similar headgasket, this may be your problem. I plan on going with a Fel-Pro Permatorque and a set of L19's to cure my problem.

If the problem isn't horrible, try a higher pressure cap like Halon said, that way the coolant won't boil until a higher temperature. I also run a 1.3 bar Koyo radiator cap and it made a surprising difference in my car.