I recently bought a 1995 E320 wagon that would randomly shutter on and off while in idle. It was like a hiccup and it would come and go. However, I could reproduce the problem without fail by idling in gear with the A/C on. It's a common problem and I've had my fair share of surfing the forums for a fix. Good news is that I fixed the problem.
For the benefit of everyone else, here's the chronological progression of what I did and what happened.
Changed Spark plugs => Didn't fix it (previous owner)
Changed Ignition Coils => Didn't fix it (previous owner)
Cleaned the Throttle Body => Didn't fix it (It was REALLY dirty and now the car accelerates smoother)
Changed Wiring Harness => Didn't fix it (But I had to do this anyway since the wires were in such a poor condition that I didn't feel comfortable driving the car. When I opened up the ignition wires, the insulation simply disintegrated in my hands and was shorting) THE FIX:
Changed spark plug wires.
After changing the spark plug wires, I've noticed a much smoother drive and more power. The old ones were the original (
) and there was obvious corrosion of the connectors that contact the spark plugs. This explains everything. The corrosion increased the resistance of the wires which prevented the plugs from getting their required voltage to light all the gas/air mixture and so the engine would occasionally misfire (shutter). This especially happened when under moderate load (A/C on and in gear). This also explains the increase in power after installing the new spark plug wires. The plugs are now getting their required voltage and so it's able to burn more of the gas compared to before, which gives more power.
Again, I know that I should have done it in increasing cost, but the wiring harness had to be changed anyway. It was a known problem with the car when I bought it, so I already priced that in to my decision.
So I hope this helps others out there who may be experiencing something similar. If you have any questions, I'll try to entertain them.