It still sounds like you have a problem with the OVP relay, or possibly with the associated wiring. However, before you go on and replace any additional parts, start with an easy thing first.

a) Check your gas pedal, its cables, and linkages. Make sure none are too tight.

b) Pull your accelerator pedal back by hand. See if that makes a significant difference to the idle speed.

If that does not work, than electrical diagnostic steps are next.

Phil.

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'95 E300 Diesel, 264,000 Miles. [Sold it]