Since it sounds like you may have resolved your problem, this isn't really necessary, but I thought I'd add this to the thread anyway for general reaction.
You mentioned that your injectors were replaced previously. I was told by an MB tech with whom I have a great deal of comfort that MB has changed the injectors that were installed in the early 300Es (I have an 86). I guess the diffusion pattern turned out to be worse when fed-mandated fuel additives began being mixed into fuels used here in the US. My memory is a little hazy, since this is a conversation that happened a few months ago, but I believe he said that the result is that there is some small amount of unburned fuel left between cycles which ignites at the wrong time or something. My 300E also has a gentle bumping idle from time to time, nothing really annoying - it has the original injectors. My tech suggested that I try the following experiment: Load up a tank of 87 octane instead of premium and see how it starts and idles and also check my gas mileage for that tank. Regular, he says, burns hotter than premium and cold starting and idle would improve. Sure enough. I also got better mileage with that tank. I'm not all that comfortable with consistently burning regular (engine heat issues - I recall that there is another thread somewhere in this forum about that), but I thought the experiment very interesting.
As a discrete procedure, just how expensive is replacing the injectors in terms of parts and labor? I do some DIY, but I'm not really comfortable with fuel systems.
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
2003 Chevrolet Suburban