P38's aren't really my bag but before tearing into the engine and taking the heads off could I suggest you do a compression test.
As you have said, it is possible you have a duff pressure cap. Have you taken it off and looked at the condition inside the cap. If you have used Rad weld, Barrs or other quick fix leak preventer itis possible the cap is blocked and can't do its' job. Also if you have used something to stop leaks I would suggest you flush your cooling system thoroughly and get it all out of there. It does seem to cause more problems than it ultimately solves as it can not fix blown head gaskets or slipped liners.
Good luck.