Idle surges are sometimes a symptom of a bad throttle actuator. The wires in the throttle actuator suffer from the same insulation decay problem as the engine wire harness. You might try and see if your cruise control works, it is sometimes rendered inop by a defective throttle actuator. Do a search--lots of info posted about this.

A bad vac leak can also cause idle problems......

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles