Temp switch?

I have a 2005 Beachcomber spa with Balboa inst. About six months ago the motor went out. I let it sit dry for that time. I recently installe a new motor and pump. Started it up, it ran great for about 12 hours got ton temp and triped the 50 amp arc fault breaker. Pulled the Thermcore heater and took it to Therm Products, just happened to be here in town, and they tested it. Turn out it was grounding out. Purchased a new heater, installed it, no leaks. Ran it, it got to 100 as i set it, the heater turned off and popped the breaker. By the way I replaced the breaker as well. Could this be a switch or sensor problem? Can i repair it or am i gonna have to pay someone to troubleshoot it?

A sensor wouldn't cause a GFCI to trip. I know this is a new heater but I shut off te power and disconnect the heater and run the pump or pumps to see if it holds. If it holds then either new heater is bad or maybe the breaker is under sized that is installed.