This is just an informational posting for anyone who might be suffering from an inconsistent idle or engine miss. My car ran great when cold, but stumbled, and the idle would fluctuate from 500-1000 RPM once it was warm. It also ran poorly with a miss up to about 1,500 RPM.

After exhausting every other possibility I found that my EGR valve diaphram had failed, which was creating a large vacuum leak as soon as the engine warmed up. I have yet to replace the valve itself, but blocking the vacuum connection from the EGR change-over valve on the front of the engine solved my idle and engine miss completely.

If you've checked everything else in searching for a vacuum leak be sure to not overlook the EGR valve.

This is on a 1993 300e W124 M104, but should be applicable to any M104 equipped chassis.