You mention going into "limp home" mode. My 400E has done this twice, both times when the ASR light went on. The throttle has a lot of dead travel (i.e. you have to press it about halfway down before any response). I shut the car off, immediately restarted it, and all was well. These are symptoms of the throttle body actuator acting up. Was your ASR light on when you experienced this? If not, I'd wonder about the overvoltage protection relay.