[Stealthee]

Mitsubishi headbolts are designed to be reusable. As long as they weren't stretched too far and were torqued to spec, that should not be your issue.

Radiator cap can defintely cause issues if the seal is worn. Fluid essentially "disappears" becasue the system is no longer sealed and it will evaporate. It also causes overheating because its not sealed properly.

I would try replacing the cap first. If it doesn't cure it, at least you are only out $7.

On that note though, it is possible to have a bad headgasket and not have leakage into the cylinders. Does the car run rough at all? Maybe try doing a compression test and post up the results.