Make sure all your contacts and contact shunts are good by making sure you have zero ohms. An Otis 20 and 50 have a GDF which is an extra Field in the Generator that is opposite polarity of the GF. The GDF is energized to decelerate the Unit smoothly. Also Mark wires and disconnect all generator field wires and check for a leak between each of the Generator field windings (GLF, GDF, & GF). Any ground in the loop circuit can cause your symptoms on a self excited GF. Could easy be a leak in Generator Interpole or Motor Interpole leaking to Ground after it warms up. A Motor shop can Meg Test the Motor-Gen. Service the GF and GDF resistors on the controller for a bad connection. PM me an email and I may have an adjustment procedure for the control.