Test the cap. FLAPS loan out a test kit which should include a tester to check to see what pressure the cap releases.
You could just buy a new cap to see if it makes a difference, but at this point, you probably like to know it a bad cap is releasing to soon rather than throw parts at the problem. (I feel your pain brother.)
What level have you kept the coolant in the radiator when the engine is cold?
When a radiator cap releases even when the engine is running normal at 180 degrees, what comes out is violent and is enough to scald all of the dogs in the neighborhood.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission