1991 300E, 150K

This saga has been going on for nearly a year now. Intermittent "misfiring" at idle in D, stumble or hesitation immediately off idle for just a split second. Smooth everywhere else, including low speeds, coasting, and highway cruising. Happens mostly when cold, but also occurs when hot. I cured it temporarily a couple weeks ago when I did the famous quarter turn clockwise on the EHA screw (see thread below). This leads me to believe the issue is fuel related.

http://www.peachparts.com/shopforum/showthread.php?p=1242138#post1242138

New cap, rotor, wires, plugs
Cleaned ICV, hoses are tight
New valve cover gasket
Checked for obvious vacuum leaks = none found

The issue is most severe when starting cold, then gradually improves as the engine warms up. But it is never completely gone, and could resurface after an hour of driving, leading me to believe the coolant temp sensor is not at fault.

Since the EHA adjustment improved things temporarily, I'm inclined to suspect the EHA or (gasp) fuel distributor. How can I test these? Am I correct in understanding that the EHA is involved with controlling idle and low RPM mixture and that this control is handled by another component (fuel tower?) at other times? If so, this would lend to my theory.

When the "misfiring" and hesitation does occur, it really feels like an ignition problem, as if spark is not getting to some of the cylinders. But, having replaced the secondary ignition components, I can rule them out. Besides, wouldn't ignition deficiency be apparent at all times and not just at idle or off-idle?

I have searched and searched and nothing seems to make sense. As it stands, I suspect EHA adjustment, fuel filter or accumulator, maybe even coil. I have ruled out O2 sensor since this is occurring in open loop (cold).

I don't want to throw parts at the problem. Any ideas?

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