You should look at the speed relays and make sure each speed step is coming in correctly. You probably have a bad contact on one of those black Aromat relays in the back of the swing down panel. This is a very common failure. Also, meter the generator shunt field when the car is running slow and compare it to when the car is running normal speed. You will most likely find the voltage applied to the shunt field is much less when the car is running slow. Good Luck!
