Jim - you might want to have the wiring harness checked for that rough idle problem. My mechanic spotted mine last year, gave me 10-12 months before I'd really need to deal with it. Apparently, there was a problem with the insulation cracking/wearing through in area's where it is very tough to visually inspect.

There have been a number of threads on the site discussing this problem. A quick search should bring a lot of good info on the topic.

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1992 500E