Help!

My 91 420SEL (98K miles) performs well except at idle. Especially during the "warm-up" period, the idle surges. You cannot really see it on the RPM gage but can see the oil pressure gage bobbing. Also, at same time I get this rough idle as well. It is not a constant rough idle but more of a twitching idle that I feel in my seat. Sometimes the idle twitch/surging gets so bad the if I am at a stop for an extended period (ie long red light), when it is time to accelerate, the engine "chugs" for a second or two and then seems to "catch" and then all is well.

I just recently had the plugs, wires, and filters replaced. The distributor was check and I was told it was fine. The put it on the scope/diagnostic machine and was told that nothing unusual showed up. I was told by one mechanic that this behavior is normal for this engine/model.
Is this true?

If anyone can give me some clues as to what to look for I would be greatly appreciated. As the least, maybe I can educate my mechanic. I would go to another mechanic but, except for this issue, he has been right on with his diagnostics/repairs.

Oh, the car is running slightly hotter than it usually does (approximately 10 deg Celsius or so).

Any assistance/advise would be greatly appreciated.