I have a 3 year old Coast spa that last summer begin sproradically tripping the GFCI breaker. The GFCI box is mounted away from the Hot Tub on the house, has a large pull out type fuse and a test/reset buttons in the center of it. I ended up having an electrician take a look at it, and he thought the most likely cause was the heater element, but could not guarantee it, since it was running fine when he was there. I ended up just paying him for his visit and I replaced the heater element myself a few days later and everything seemed to be fine, went all winter long without any tripping. Now propably 2 months ago it begins with the sporadic tripping again, it runs fine while I am out there, cycling the pumps and such, but when I come back out the next day it is off. I disconnected the lugs from the heating element, to see if that was the issue and it seemed to run for a few days, then began doing the same thing again. I had somebody suggest unplugging the ozonator, which I went to do, only to discover I don't have one, so pretty sure that's not the problem. Anybody have any other suggestions, of what my next course of action should be? Is it likely that it's the GFCI itself that is the problem? Thanks in advance.

could be any number of causes
your electrician should be able to test prperly with an insulation resistance meter to determine whats causing the earth leakage. it may be difficult to find being intermittent.
you can try and narrow it down by only plugging in the essential equipment - ie the filter pump - with everything else disconnected that limits your problem to one pump, heater, electrical supply circuit.

Be sure your breaker's not getting hot -> bad connections at the breaker one of the biggest causes of sporadic tripping.

I did notice a humming sound coming from the breaker box, is that normal? How do I determine if it is getting hot, kill the power and just look at all the internal connections for signs of overheating?