I'd also consider replacing the main engine wiring harness, if you can't verify that it's been done in the past few years. This part can be purchased wholesaie for around $550, and is a several-hour DIY job which is not rocket science.

I had an occasional miss that would come up at idle, particularly when the car was warm. The car would shudder when this would happen. Replacing the harness removed it for me.

If you don't know, this harness is one of the only known weak points of these cars. They were made out of a biodegradable wiring insulation during the years 1993-1995 for all MBs and age and heat (especially with the packed 500E engine compartment) tends to make the insulation dry up and crack, or worse flake off, causing wires in the harness to short out/together. This leads in bad cases to all sorts of weird running conditions, or as in my case merely a mild miss. I was shocked to see the degradation in the harness of my 85,000 mile 1994 model.

You can see far more about this issue and other problems at the mecca for 500Es, which is at http://www.500e.com

Cheers,
Gerry