A customer had self installed a new Hot Springs PDR (Power Down Reset) heater in his Hot Springs Grandee a year ago, and this happened:
The actual cause of the problem is that the heater power wires were not fully latched by the grey power block, and with the heater drawing in excess of 23 amps, created the effect of a bad connection. This bad connection generates heat, and gets worse with time.
Since the heater circuit board was relatively new, so it was repaired on site by soldering new wires to the board, bypassing the connection block.
With the repaired spa heater board reinstalled, the heater power wires were reconnected to it with large wire nuts.