I had a similar problem- dipping idle at stop lights. It started with a mild dip on startup, which got progressively worse. I changed the fuel filter just as a part of routine maintenance (It had about 30.5K on it, and had seen some tough duty cross country at slightly shady looking gas stations), and this cleared the problem right up. I still get an very occasional and mild dip at a stop, but is extremely rare (I think I've noticed it once in the past month or so).
98 e430, 55K