[TGPilot]

Run a coolant bleed-down test and a cylinder compression test. One thing I had happen to an import car in the past was the radiator cap gasket swelled where it met the top of the radiator neck. With it swelled it was a very tight fit in the bore of the neck. The cap would try to relieve a little pressure but then get stuck open slightly. No pressure or lower pressure in the coolant system causes the boiling point to drop off drastically and boil over. You might check that along with the other suggestions...