Symptom was: When letting off accelerator, engine loses injector signals and spark until RPM falls below 1500 regardless of throttle position, then reacecelerates fine.
Found: Unplugging throttle switch made no difference to idle speed as it should (switch calibrated fine), found 5v on full throttle side of connector (correct), but zero on idle side (should also be 5v). Resisitance (with computers pulled) on idle side infinate (no shorts draining off signal). Other DME computer (substitued) showed no change. Somehow no voltage on idle side tricks computer into thinking that the engine throttle is always closed which causes the symptoms above (closed idle switch reduces signal to zero as it now has all the time).
I introduced 5 volts on the idle side of the throttle switch lead buy connecting a 47 ohm resistor between switched 12v and idle switch lead and bingo-the car runs fine with normal acceleration and rising engine speed when the throttle switch connector is pulled.
Question is: Where does the 5v signal come from normally? I assumed it came from the DME internally, but substituting another box did not change anything, so I am doubting this now. The Bently manual doesn't show anything but a jumper connection between the Motronic, the Idle Speed Computer and the throttle switch indicating the 5v must come from the computers internally.
I am tempted to leave the resistor in the circuit, but fear that the introduced voltage could damage the DME computer.