A faulty reservoir cap can allow even normal systems to build excessive pressure.

If your cap tests normal (venting avove the rated PSI of the cap), it is probably a case of a lot of exhaust gasses making it past a bad head gasket (and warped head??) into the cooling system, especially if it happens even before the engine is up to temperature.

You can buy test strips that when dipped into the coolant will verify exhaust chemicals, but if you are building pressure faster than even a good cap can vent it, I suspect you should be able to see bubbles in the coolant with your naked eye.

Bob