I can't believe that is the 'reason'. The radiator CAP does not sense temp.. only pressure.
First, the pressure cap on the radiator would have have lift in order to allow the water to come out.. that will not happen unless the system is at 3-5 (or greater PSI).
I'd take a serious look at the t-stat housing for something blocking the normal t-stat operation.. a casting bur, some slag, something is causing the t-stat to stick. And the built in by-pass hole is large enough to equalize any pressure differential... the build up in the block due to water expanding is not that quick.
Yes, drilling a 3/4in hole in the t-stat allows the water to circulate, but at this point you might as well just take the entire t-stat out.
