Since it trips frequently, for testing purposes, I would try to
seperate off the internal loads one at a time and temporarily insert a
seperate GFCI in each load just for testing. On mine, I could easily
split off the pump, power/pack/controller, air blower, all of which are
120V loads and can be temporarily wired to a std GFCI. If you know
what you're doing and are comfortable working with elec, this isn't
hard to do. Then you can isolate which load is causing the fault.
While doing the testing of course, I wouldn't use the spa, nor would I
use it till I figured out what was wrong. I'd start with those, since
you've replaced the heater, it's unlikely that's the fault.