I'm guessing there's a possible vacuum leak causing the idling vibration & the fuel pressure regulator or accumulator for the morning starts. But these are just guesses. With fuel injectors going through a minor rebuild; I doubt it is a leaky fuel injector.
After reading 100s of posts, I did notice one of your posts said if the air mass sensor failed (or is failing???) then the idle could oscillate. The idle is oscillating but not enough to notice on the tach, but you can hear it alittle. While giving the car alittle gas (foot on brake and car in gear), the oscillation is easier to hear. While doing thing, I'm giving the car just enough gas to to raise the oscillation noise/vibration - but the tach is still pretty much at the same 500 RPMs. Can air mass filter gradually decay in performance, or is dichotomous - when they fail it is all at once?
One thing i did notice, the RPMs don't change whether the car has the AC on or off. Is this normal (I expected the RPMs to be alittle higher with the AC on).
While in drive, if you give the car just alittle more gas (the RPMs barely go up - if at all) then the vibration goes away. What influence the '95 420's idle speed? While driving, the car feels normal. So, it sounds logical to me that it something control the idle speed, or something providing an input to the idle speed.
I'm going crazy with this (alittle broke too!).