I don't know what happened, but the car is smooth as glass at this point, both warm and cold. Not even a hint of roughness when warmed up - symptoms are ALL gone. RPM's are a touch higher.
Here are all the things I did since I started this thread:
1. Checked all vac lines - nothing
2. Added oil additive to clean out valve train
3. Added injector cleaner to the fuel
4. Went to the idle control unit. Removed it. Sprayed the bottom contacts and the plug with Radio Shack TV tuner cleaner
5. Thoroughly cleaned out idle air valve with brake cleaner, and lubed it with silicone spray. It needed it.
6. Re-checked plug wires for the umpteenth time. Found one not 100% pushed in (oops!)
Well - live and learn. I know it could not have been in the plug wires originally, because the problem existed when I bought the car, and I then replaced the plugs and wires. But still, it COULD have been No. 6 above as a new cause. Again, as we say in IT, one problem - you'll figure it out, two problems - you'll go crazy.
Still, I am going to look for a fuel filter. Sounds like a good piece of preventative maintenance, and that fuel distributor is expensive.
Anyhow, this car runs like a true charm. You can hardly tell the engine is running.
Maurice - thanks for all your help. I don't think I would have tackled the idle air valve without you telling me how to do it, and it was a simple thing to do.
Another resolved issue on the mercedes shop forum!!!
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)