My 93 945t idles at @ 550 RPM and will sometimes hesitate at idle. The problem is more pronounced when the car is in park, The idle will rythmiclly surge between @ 450-600rpm. I have a few ideas what might be causing the problem and so far have eliminated a few possibilities such as:
New Bosch cap & rotor
New NGK std plugs (old plugs looked O.K. possibly running a bit lean but not too bad)
New IPD wires - tried swapping back old wires with no change
Tried 2 different AMM's both cleaned - no change
Tried 2 different idle air valves both cleaned - no change
Cleaned TB - all new vacuum lines - Adjusted TPS
Checked for vacuum leaks and found none
FPR & ECT sensor are new
The car isnt setting any fault codes
Has a less than 1 year old O2 sensor that was a universal 3 wire Ebay special

Also now that the weather is warmer I've noticed that while sitting in traffic whenever the electric cooling fan kicks on it will momentarily drop the idle to where it almost stalls but will even out back to @ 550rpm in a second or two.

My thoughts are that the cheap o2 sensor could be the problem or maybe the ECU needs to be replaced, Or possibly a low voltage problem. But to date most of the ideas I've come up with havent helped much. I'd like to quit throwing $$$ at it so I hope this is enough info for a more educated diagnosis.