As one poster mentioned it could be any number of things.
Some things things to be checked with e-unit itself.
Wires loose in there solder joints in e-unit and including other ends of wire going to field , roller pickups, ground etc.
Drum distorted, burnt spots on drum contacts or its contacts dirty .
Reason why one can turn engine upside down and back up right side and have it work again as the drum will advance to next position and sometimes work again until cycled around to that same contact.
Top and bottom E-unit fingers not making proper contact with drum.
E-unit Lever loose not making firm contact with its rivet on the e-unit board.
E-unit plunger sticking in coil not moving freely.
you mention-
Just out of curiosity, is there anything in the voltage from the transformer or track system (ie. shorts) that could cause a malfunction in the E-unit?
Yes a simple type of bump going over switches can trip a problematic e-unit into neutral or make it silent if one of the wires is loose in the solder joint -making sporadic contact.
or
short on track will trip e-unit.
For the patient person that has experience with them its worth the time to trouble shoot and fix otherwise replacement can be made.
Since you brought it in for repair they may have just sprayed it and perhaps should go thru it but it may cost more than a replacement in the long run.
