Try to check if there are any pinhole leaks in the hoses. Sometimes, a leak is only evident when the pressure gets high enough.

The cap is also an easy fix. As is the thermostat... Though I doubt it's the thermostat... It would be apparent more of the time.

But since the car has been newly overhauled, I'd also take the time to check if the head bolts are torqued properly and that the engine has good compression when hot (not cold, you already know it runs okay when cold) . Sometimes sudden bursts of high temperature can also come about from the head lifting under stress.

A simple way to test this is to open the radiator cap when the car is cold, turn on the engine and run it for a bit. If there's a lot of bubbling under revs, go back and have them check the workmanship of the overhaul.