Build the home brew tester at the very bottom of this link, pull the codes and use that site to determine what the codes mean:
K6JRF Auto Page
The Electronic Throttle Actuator (ETA) controls the Idle Sped Control, but if it's the original harness, I think most people here would agree that it is most likely in need of replacement. I would start with the harness after the cheaper stuff like the fuel filter, fuel pressure regulator, stop lamp switch and neutral safety switch.
Incidentally, I had a 1993 400E w/out ASR and I remember that for some strange reason my idle got lower and lower over a period of several years. So low that it occasionally stalled while moving from reverse to drive; but after changing the motor mounts the problem totally corrected itself. How the two were related is beyond me. Maybe it was just a coincidence.
I think all the 400E's were LH (hot wire).