I'm with QWKSLVR. A 7lb. cap! You must be loosing coolant. You need a 16lb. radiator cap for sure. I purchased a 79 Z28 years ago that had a 13lb. cap. (owner didnt know why it was overheating) When I would turn the car off I would here it purge/release from the cap a few times before it cooled off filling my overflow. This could be one of you problems.