1993 965 216,000km.
Once my car is up to operating temp I'm getting an intermittent fluctuation in idle and a stumbling when maintaining a consistent speed. There is also a hesitataion on acceleration. Runs fine when cold and while warming up.
I first noticed this on the highway at 120kph in cruise. Engine felt like it was surging, very slightly, but enough for my passenger to comment on it. Then when we got off the highway and were pulling up to a stop sign the tach dropped almost to zero (didn't stall) and began flipping between about 200 to about a grand. I put the car in neutral and reved the engine once and the idle smoothed for a second or two, then started doing the same thing. I can feel the fluctuation at 50kph(30mph)in city traffic also. So far have replaced plugs and cleaned throttle body. Any ideas?

Cold be a thermistor problem, the one sitting à the rear base of the motor head... When this thermistor goes, so goes any valuable information on operating temperature going to the engine computer. Generally experienced also with hot-start problems.

Good News!!!! I finally got my car back. Working at a dealership automatically relegates you to the bottom of the list and we've been busy(a good thing)so it has taken a while. At least I was lucky enough to have loaner cars to drive home at night.
Enough about that. It was the oxygen sensor! Not what I expected since it was replaced when I bought the car three years ago. It should have lasted longer than that. Intermittent voltage spikes was the problem.
Next project is to find out what is causing the engine to run rich. The O2 sensor was BLACK! So were the plugs we replaced earlier. Now, it's entirely possible that the sensor has been bad from the beginning and that's what caused it to run rich.
Suggestions are a fuel pressure test and a vacuum system check. I'll post anything we find.
Donn

Leaking intake gasket, not unusual for this engine?
A bad coil that can't handle the heat from warmed engine?
Power stage that is failing the same way?

Try to determine which cylinder is missing.

DanR 94 964 328,000 miles (94,000 on the new engine)
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DanR

Thanks for the tips. The engine doesn't have a miss, there's no smoke involved,
and there are no codes set. It's an rpm fluctuation and it's driving me NUTS!!!!
Donn

Sounds like a good list.

May I add that a failing crank position sensor often acts up when it gets hot. That would actually have been my first guess. Rather common, especially if the cable sheath down by the sensor gets damaged, chafes against the pipes, or is crispy. That unfortunately doesn't set a diagnostic code unless the sensor signal is totally severed. It would at least be a little less expensive to deal with than some of the other possibilities mentioned.

I'd also add a cracked distributor cap or bad ignition wire to the list, although I'm guessing those might misbehave just as badly when cold as warm.

There may also be a concurrent problem at idle such that when the engine is fully warmed, the idle air control valve operates in a zone where the piston is sticky (from wear or dirt). A dirt throttle body throat/plate or misadjusted throttle body wouldn't help that situation (but you shouldn't need to touch throttle adjustments unless you've previously been tinkering in that area). That could account for the bad warm idle.

If a cylinder is missing (where would it go?) then that would be very indicative. Generally, a single fouled or misfiring plug would be worse cold rather than hot, so I actually wouldn't expect you to see much variation based on your symptoms. However, if you do find one or two plug tips that are fouled or significantly darker or lighter than the rest then that pretty much narrows the problem down to either a) the ignition circuit from its distributor contact (including a cracked cap) to the plug tip (like a bad plug wire, very poorly gapped plug or even a bad plug), b) to a fouled injector (run the OBD diagnostic as per the FAQ to exercise it and/or use an injector cleaner additive), c) to blowby in that cylinder or d) a valve problem.
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Dave -940's, prev 740/240/140/120 Never overestimate what little I know.

Thanks Dave. We can eliminate the cap and wires idea 'cause the 960 is coil on plug. The chafed crank sensor cable is one I'll be exploring!
Donn

Too add to the good responces you've received-check all of your vacuum lines closely-idle control valve as well-but my bet is the intake gasket knowing the model of you car-they upgraded the gasket in 96-the ones made before it's just a matter of time before they go.
Poolman

Thanks Poolman. Car has the updated gasket on it but we did check for leaks anyway and it's OK. Will be checking all vacuum lines.
Donn

Perhaps your problem is in the gas delivery system. You may want to pull the fuel filter and check that it's not restricting fuel flow. The next possibility would be in the pump/reg/piping system.

Let us know what you find.
'97 960 117k miles

Thanks, we haven't looked at that yet. I will post the solution when we find it and I'm sure we will, eventually :-(
Donn