UPDATE Finally got the car back. turns out it was a bad casting on the head. The water jacket (a hole that passes coolant from head to block) was not cast all the way through. It was close, tech said he could have tapped it with a screwdriver and broken through, but it was enough to stop coolant and hence start overheating.

So car is running great, only 400 miles on it, so haven't even stoped on the gas yet.