Well since the thermostat is closed when the motor is cold you should get no flow to the radiator so even if it was clogged it would not cause the cap to let out pressure.

So if the T stat is closed cold you either have a bad radiator cap or combustion pressure in the cooling jacket. If your cooling hoses get hard very quickly on a cold start its probably a cracked head. The head crack seals when the motor gets hot as the chamber expands.

Typically if you have a head crack one plug will read differently and you will have a miss at cold idle.

Kevin