I have an intermittent idle speed problem. 1986 300SDL, OM603, 245k miles

Symptoms: sometimes at idle, the tach will erratically jump up and down from 700 to 1,000 rpm and waver, never steady at any one speed. It's not just the tach reading funny, the engine speed is changing. I can feel the engine rpm surge, there is chassis torque reaction in gear, and I can hear (and watch) engine rpm variation in neutral

Sometimes the idle steadies out by itself. Other times, after fluctuating for a bit, the tach drops to 0 rpm and the idle slows, probably to the mechanical limit. I assume at this point the EDS has given up and disconnected itself. When I apply throttle, at about 1,000-1,500 rpm the tach again registers. The engine will then idle at 700 rpm and the jumpy idle may, or may not return.

Cold light-off is a steady 700 rpm with the few hiccups normal to the 603.

Question: is the EDS adjusting the IP in response to an erroneous idle rpm signal, or is producing the adjustment on its own? I have not been able to deduce from the wiring diagrams on the CDs if the EDS drives the tachometer, if the tachometer drives the EDS, or how they're related.

Related symptoms? The speed control will occasionally pulse, sometimes a speed-up pulse, sometimes a slow-down pulse, sometimes erratic up/down movement. This may occur a few times during my 3 1/2 hour commute, and occasionally the up/down pulses will cause the speed control to disconnect.

I usually set 65-70mph as cruise.

When it is acting unstable, if I disconnect by tapping the brake, the Resume will function and the pulsing may, or may not be back.

If it disconnects itself, the Resume will usually not work.

Once, Resume produced an erroneously high resumed speed. I don't know how high it wanted to go, since I decided when we reached 90+ with no sign of limiting that it just wanted to air itself out... The car does like to run, though...

I don't know if this is related, but I have to wonder if both systems have a common speed/rpm sensor problem. Again, I have not found the answer yet in the CDs.

Please note: these problems are intermittent, so nothing has completely failed at this point. I don't want to start removing and replacing unless I can prove it's failed...

The car remains completely driveable, it just acts funny at times...

I love intermittent electrical problems...

Thanking all in advance for help.

Jim