I HATE replacing parts as a means to solve a problem. Especially with M.B. ($$$$$) parts. Your problem COULD be the OVP relay, or it could be a decel switch, a bad air idle valve or a throttle valve switch or even a crank sensor. All inputs are needed for a smooth, steady idle.
I think we all agree that Stevebfl policy of testing parts as opposed to replacing them is the way to go. If your OVP has two fuses on top, its probably the latest version and not the problem. But you could test it by applying 12v to pin 30 & 15 and grounding 31. Check the voltage outputs at all the other pins (87E & 87L). Outputs on these pins should be near battery voltage. Wiggle and lightly bang on it while you are doing this to see if you can get it to short.