Follow up with my E320. I finally got it back from the dealership. I guess the problem all along was over- and under-voltage spikes. These occurred too often, so the computers finally put the car into limp-mode. They said the ASR system is the most sensitive to this, so it's the first to react.
A simple clearing of the computers with a dealer scan tool, and I guess I'm back on the road. It looks like this is something I couldn't have fixed myself...