I'm hunting down an unstable idle on my '95 E420. I know that this question has been posted well over a million times on the forum but I'm pretty much in the dark in determining what to check from here. Here's a quick rundown:

The idle problem is too subtle to throw any codes and doesn't fluctuate the tach at all. It is at its worst in P or N. When the car is put into Drive the idle becomes almost like glass but every so often you'll get a little bit of the symptoms that you experience when the car is in P or N. The car just doesn't feel smooth and once every couple of minutes the engine will hiccup, almost like its adjusting the fuel mixture. If you rev the engine in Park up to 1500-2000 rpm and then let off the gas and let the engine drop, the engine's stumble becomes more noticeable as the tach settle out.

At this stage the car feels like it's running too lean or there is a vacuum leak somewhere. (or both for that matter) Aside from the idle problem, the car seems to run fine. No hesitation during acceleration or any larger symptoms. Here are the specs on the car and the parts replaced/checked.

-The car has 100,000 miles on it
-New wiring harness at 90k.
-New Caps, rotors and plug wires and plugs (all Bosch)
-Ran 2 tanks of Techron (can you purge the injectors under pressure?)
-Replaced all vacuum hoses in engine compartment (intake, egr, etc.)
-Sprayed down all breather and vacuum hoses with carb cleaner following their install to ensure there weren't any further leaks. Also sprayed down each injector to test for any seal leaks and didn't get any results.
-New O2 sensor installed last month.
-Cleaned out throttle body to ensure a clean throttle plate.
-Replaced boot on the MAS to ensure that there weren't any cracks causing a vacuum leak. Also disconnected MAS to see if the idle improved; no result.
-Checked vacuum hoses on Fuel Pressure Regulator to ensure there wasn't any fuel leaking out to indicate failure
-Listened to both fuel pumps to ensure both were running

Any tips on where to go from here would be most appreciated! Thanks again!

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL