Been having problems with coolant boiling up and out of the cap on my '06 GT after I drive it longer than ~10 minutes.

Tried to use the Autozone coolant pressure tester, but every time I try to use the tester's pump, the pressure never holds and I can hear/feel air hissing up out of the reservoir where the cap would be...no matter how hard & carefully I tighten that pressure tester fitting air just hisses out.

I've never heard of the threads on a coolant reservoir going bad...has anyone...

Crazy coolant reservoir tank '06 GT