Bummer, was hoping you'd be in Hot Water by now!
Probably the circ pump... does the spa have an ozone generator?
Try unplugging the circ pump, and see if the problem goes away...
I suspect the circ pump may be having more issues than current leakage... if the heating element is getting warm to the touch... it should remain approx the same temp as the water (approx 4 F warmer).. So, you may just be killing two birds with one stone by replacing it
Just to clarify, you removed both leads from the heater while testing?