I too have a similar problem with my '89 300CE. The slightly rough idle is definitely noticeable. So far the fuel pressure regulator, injectors+seals, fuel filter, new Beru wires, and new Bosch plugs have been installed. I also visually inspected the cap and rotor which were both in good condition. The dealer sold me new Bosch Copper plugs (HR9DC) as I can't find the original H9DCOs anywhere. It appears that stock has run out of the originals.

The NKG platinums I pulled out were in excellent condition, no fouling or any other indicator to a poorly running engine. Just out of curiosity, I've noticed that the economy gauge tends to sit off the peg and inch towards the red area at idle. I can see it fluctuate slightly at times. The tach also moves up and down ever so slightly as the engine surges lightly. When I put the car into gear, such as reverse or a drive gear, the fuel economy gauge moves well out of the black area and towards the right. Is it normal for it to do this when the engine is put under load, even when no throttle is being applied?