I'm not a mechanic at all... But sometimes the solution is extremely simple. My 1990 300ce was having lots of idling problems and I had a million things replaced. I spent over $7k on a laundry list of stuff. The idle problem went away for a bit, but came right back after 1,000 miles.

I sent some injector cleaner through and the car actually ran much much better after that. So the next time I went in for an oil change at Jiffy Lube and they tried to sell me their fuel-line cleaning service, I took it. My car has 190k miles or so... Apparently everything the fuel runs through gets cleaned. I changed my fuel filter, too. When the ran their cleaning mechanism, my car sent white smoke everywhere, for a couple of minutes. It was crazy! Their whole service bay was smoked out and the employees had to step outside. Apparently, my combustion chamber, etc was caked up with crud. Now, when I start my car, I can't even tell that it's running. When I accellerate, it doesn't hesitate at all... The improvement was far greater than what I noticed when I had the mechanic tear everything apart and replace nearly every damn thing under the hood!

Give it a try! I've also heard of a similar thing called BG 44k from these forums. Good luck, I know how frustrating this sort of thing can be!!