Sounds like you are down to a partial short in the exciter coils or rotor coils. Good chance its going to be a rewind or toss it.
One method to possibly confirm it would be to put a full wave bridge rectifier on one half of the exciter windings and power the rotor off of that and see if it does it. If so then switch it to the other side and repeat.
If it still does it you may have a partial short in the rotor windings. Then the only other option would be to externally power the rotor and see what happens. If it drops out just the same with an external power source its a rotor problem.