Motor field coils, or the voltage to the coils. If the fields are weak, you can over speed. Or have you checked the governor with a tach? I f the 90% switch isn't set correctly you may think your over speeding.
R6 resistor parallel to the shunt field could be open. This resistor is there to slow response. If it opens, the shunt field equivalence goes up and draws mor voltage. The car wont overspeed much, but enough to trip the gov.
