I had a problem last year and was advised to switch the wire on the magnetic flow switch so that it wouldn't cut out. This worked great for over a year. I just changed the water and filter and the tub circulates fine but when the heater cuts in, the GFI trips. I tried switching the flow switch wires but no luck. The tub worked fine before the change, so I doubt it is the elements but am not sure. I could just replace both but would prefer to know for sure. When the thermostat is turned to where the heater switch cuts in, the solonoid switched, which send current to the elements but as soon as this happens the GFI trips.
Any advice would be appreciated.

Disconnect both the heater element wires, and try your spa again. If it holds with the element disconnected, you will need to replace the element.