DC Generator Out of Control

What would make a DC generator (GE 752) with exciter field regulated by a basler regulator which reacts to a 24v throttle signal suddenly go to full output with the throttle signal steady, measured field coil 2 ohms checked everything for ground faults ok, voltage feedback is via 2 80k resistors they are ok. Changed regulator still the same problem, fitted the first regulator to another generator and it works ok