I also have a 1992 400e. Last year, my wiring harness gave out and I had very similar problems.
First off, the wiring harness is a know weakness on the M119 engines. The insulation tends to get brittle and crack and wires start to short out. This harness controlls most of the engines electrical functions and sensors. The engine temp sensor is one of them.
The car was driveable, but it would buck and stall. So I was able to get home.
Check the harness first. It is located on the passenger side of the engine compartment in the rear corner. It goes from the main computer housing and contains the fuel injector connectors, throttle body connectors, distributor connectors, and the temp sensors up on the front of the engine.
You can just follow the harness to an area where the wires are exposed, or better yet, CAREFULLY cut into the rubber covering where it enters the engine bay, right by the valve cover on the rear of the engine. I will bet that the wire insulation is as brittle as glass.
Do a search on 'wiring harness' and you will see my original post. If this is the case, you may be able to get MBUSA to cover the cost, but I was not that lucky. The harness cost about $500. I installed it my self in about 15 minutes. The car ran fine afterwards. The only damage was to my temp sensor. It was shorted out, probably due to a spike.
Good luck. And please update with what you find.