RPM drop when stopped and ingear

My 1995 E320 wagon with 79,000 miles will shudder and shake when stopped and in drive with brake applied. The tachometer visibly drops off a few rpm's.

My Mercedes mechanic called it "a misfire under load" and blamed it on old spark plugs and at 75,000 miles repaced them. The problem did not go away and at 78,000 miles he blamed it on 2 faulty plugs - hard to believe. With this fix, the problem reappeared almost immediately and seem to get progressively worse.

The car always starts properly, has never stalled, the engine light has never come on and once up to speed, rpms hold steady and the car runs smoothly. While running in park or neutral, the problem is not noticable.

I should note that I purchased this car used with 67,000 miles on it last year - the used car dealer told me that a wire harness had been replaced as part of a recall.

I am going to give my mechanic one more try before going to
someone else.

Any ideas?