strangely enough jig, mine is doing exactly the same thing.!
My shower stopped heating up so the landlord came around and put in a new something, anyway it works but like you said only for two or three minutes, which is particularly annoying when your washing your hair or shaving your bits!
Anyway my downstairs lights blew its fuse and since then the shower has worked properly, but then i put a new fuse in and low and behold the shower kept tripping its switch again.
The end result is im clean but sitting in complete darkness!!