I have pretty much the same story except that since Nov04 after a dealer visit, my starting slowly got worse. It used to be OK on a cold morning start but now, cold or warm I got the same issue. I tried fuel injector cleaners and leaving the key in #2 position for 10 seconds, no change, which is why I am on "the site" now.
I'll let you know if I get anywhere and i'll watch your post. I am leaning towards the fuel filter, check valve or accumulator, but I hate to just through money at it.
Steve

I should of added...I had it in for new plugs, caps, rotors, sparkplug wires, and a few new vaccum lines to fix a rough idle problem. It is a W140 1993 500sel.