Maybe the surging shook something loose inside the genhead. Did you go thru and solder all the crimp on terminals on the wire ends? The crimp on terminals were terrible quality and poorly done on my ST. I soldered every one of them to keep them from vibrating loose from the wire. What type AVR do you have on yours, or does it just have a voltage control variable resistor on the field circuit? most ST's don't have an AVR. Since it is putting out some voltage, that most likley means that the winding circuits are in tact. Check all the electrical connections around the AVR circuitry.
If I had this type situation on my harmonically excited ST head, I would suspect that one of the 4 rectifyer diodes had failed or come loose and I had lost some of my excitation current which would cause the lower voltage.
Ron