Your stalling condition has one significant symptom that most 300E stalls don't have. That is the fact that the engine was gone at 40mph.
Based on that I would guess a fuel pump relay to be the problem. This symptom (stall at speed) eliminates the OVP relay as the cause or any other part of idle control for that matter.
Unfortunately the intermittantcy of your problem could be also caused by a couple ignition problems also AND none of it can be tested. I can't tell you how many hours I have spent driving 300Es with the front seat full of test equipment monitoring fuel pump signal, crank sensor signal, coil control signal, speed signal, fuel pump pressure etc, etc. When its working it ain't broke!
[Edited by stevebfl on 12-12-2000 at 08:40 AM]
Bosch Master, ASE Master, L1
33 years MB technician