I'm running a 93 400E also, I've experienced similar problems. I would strongly recommend replacing the upper engine harness first . You won't beleve how much deterioration has occured with the engine heat in that engine compartment.

It's a DIY job, I started at the CAN and replaced the new one as the old one was removed. You'll be surprised at the amount of degraded wiring that's living in there. It sure can cause numerous running discrepancies.Definitely do the harness.Good luck.