Your OVP may be causing this. It was in my case. Can't say about yours. Remove it, then shake it. If you hear rattling, replace it. This is not a conclusive test for OVP functionality, but it's good for a start.

This device provides power to the idle control circuit. It has other responsibilities; I'm just trying to show it's relevance in this case.

__________________
'91 300-SEL