There have been numerous posts on your exact symptoms. I would search using key words EHA, OVP, fuel pump relay, stalls, etc.
From a diagnostic point of view, one must monitor the current flow on the idle speed regulator and the EHA and probably the crank sensor should be monitored on a scope. You need to see whether you loose fuel, ignition or just run too slow. There is a service bulletin that applies to 103 motors in 124 bodies that raises the idle speed (it may also apply to 126 cars).
All these tasks require special tooling and information. If you want to start throwing parts at it I would start with the OVP relay.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician