I'm working from memory, but I believe the brake switch, located above the brake pedal, can go flaky and affect cruise control. Also, if the trunk mounted brake light isn't working, this may affect cruise control.

If the engine wiring harness is still original, then I wouldn't start diagnosing any such problem until that is replaced. Same goes for the throttle body wiring.

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95 E320 Cabriolet, 169K