Its only a wild guess but I gather your injectors might need either a very good clean or to be replaced altogether. Then it might also do some good to check on those spark plugs and get an electronic-tuning done.
My colleagues' car had such symptoms as yours, and the above rectified it. We sent it to a tuning specialist where he tuned the engine, and also something down the driver's compartment supposedly for the fuel injector.
The next thing my colleague knew was that his MB performed better than ever.
I hope your problem will be sorted out soon.
126 tailed by a 203, 129 leading the pack.