1990 420SEL idles rough
 

just recently, my 420sel developed a sporatic miss at idle. it isn't constant, but it's there. I replaced cap, rotor, plugs, etc., no dice. I have heard the EVP might cause this. I shook it, and it rattles. I'm not familiar with what this part is, or what it does. I've just read about it here. would a bad EVP cause a sporatic miss at idle? where do I get a new one, and how much do they usually cost?

The OVP (overvoltage protection relay) will cause a hard start but your problem is most likely fuel injection related. Check injectors by cracking lines individually and see if the miss disappears.