How can i fix my triton opal II?

The unit keeps tripping the circuit breaker on the normal setting, it works fine on the cold and economy settings but on the normal setting it runs for about 30 seconds and heats the water then trips the cb, i have checked the inlet filter and water pressure and they are ok, i did notice a slight pulse in the flow after about 20 seconds then the unit tripped out, i have checked both pcb's for dry joints and faulty components and cannot see anything obvious. I suspect the thermal cut out but it will run on economy on 10 and gets very hot and not cut out, the unit is out of warranty now and the missus is starting to complain so any help would be greatly appreciated. Thanks.