Yep I'm starting to think the head gasket as well. It was just replaced about 2 years ago, and maybe 2K miles. It was done correctly and the head was reconditioned and milled flat. Studs replaced and head bolts re-torqued after being ran.

What happens is the motor will start running at fast idle and you can hear the coolant begin to boil into catch can. If you are driving the motor will cool down after you get up to speed about 30mph. As soon as you stop though the coolant will begin boiling after that from then on. You can even poor cold water on the radiator, and all it does is boil off. The coolant inside is still boiling out the catch bottle. If you let it go too long it boils out enough coolant the motor could be damaged. Don't think that has happenned yet, althought it did get pretty hot one ride.