Erratic Idle problem- om603 ...!?
 

I've got this peculiar problem with my idle... The car will idle smooth as butter (aside from the tappy lifters :-) ) most of the time, real quiet and healthy right around 600rpm. If I idle my car for a long time or come off of the highway and come to a stop so the engine is idling, the idle will flutter up and down and jerk all around in +/- increments of 200-300. After that happens for approx 2 minutes, the idle will drop to 400rpm and violently shake the car. It's real aggrivating. I can rev the car up, there is no loss of power, no smoke, no misfire, no odd smells or terrible noises, just a real slow and "sticky" obnoxious idle. What could be the cause of this? Once this idle problem occurs, it will go away within 2 minutes of driving the car. It only happens when the car is up to normal operating temperature

Another instance this occurs is if I have my foot on the pedal holding a certain RPM for more than a minute with the car fully warmed up, and if I'm driving in slow traffic without stopping and have the RPM's driving at one RPM for an extended period of time. I feel like it's not good for the car and it sounds to me like the sound of money :(

Has anyone experienced this in their 1987 300D (om603) or similar?

Start by changing the fuel filters, I've had this from clogged big/primary filter.

Fuel filters
 

Ditto,

This has happened to me (biodiesel rinse of diesel tank). After changing the fuel filters it went away. There is no ignition so it has to be fuel. Start with the cheapest parts first:

Check lines. Replace primary FF, secondary FF, tank screen, injector nozzles, lift pump, (sell?) injection pump.