The OVP and leaky EHA are more likely the problem than what I'll suggest; here it is anyway:

Fuel injectors leaking down into the cyls. Normally, this results in the car idling raggedly when restarting warm and the car has sat for a few mins., and not when you've stopped, got out and immediately restarted.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"