Need help diagnosing varying RPM at steady throttle when engine is warm

[bahassler]

1983 Pininfarina FI Spider 2000 is the subject car. I recently took all the steps required to get it running after sitting for 30 years. This included new gas tank, radiator, thermostat, hoses, fuel pump, fuel filter, fuel hoses, spark plugs, distributor with new magnetic pickup, ICM, plus refurbished injectors. I have also cleaned the auxiliary air control valve and checked for vacuum leaks.
This refurbishment took place over many months in stepwise fashion noting improvement at every juncture. I am now at a loss for what to investigate next to fix this last bug.
The car starts fine, idles fine and revs strongly without hesitation. The problem begins as it warms up to operating temperature. I hold the throttle steady buy inserting a small wedge between the idle speed screw and stop so it isn't my foot being unsteady on the gas pedal. The engine RPMS hold steady and then drop slightly momentarily and then resume at a steady level. This phenomenon repeats itself every few seconds. It doesn't matter at what RPM I try to hold steady. The characteristics are always the same; even at an idle.
Many people probably wouldn't even notice this but I know that something is wrong.
Can anybody suggest what I could check/test next?
Thank you.

[bahassler]

Solved: Problem ended up being the oxygen sensor (lambda probe)

[rvwinkle]

Thanks very much for your post. I've seen some of the same problems with my 1980 fuel injected spider. Presently it's up on jack stands to sort out the brake problems. Anyway, did you have any issues removing the old oxygen sensor?

Thanks,
Lee

[bahassler]

Lee,
Surprisingly, after 40 years, I did not have any problems removing it. All work is done from the top side of the engine bay. Getting the air intake tubing and the exhaust heat shield out of the way makes for easy access. A 7/8" box wrench worked nicely. I bought the car new so I know it's never been changed. The phenolic insulator on the spade connection to the ECM broke into many pieces while trying to separate it from the O2 sensor lead. I suppose it was due to age and engine compartment heat exposure. I just wrapped several turns of plastic electrical tape around the male spade end; not the best solution.