Rob - what kind of traffic were you in when the problems started. On my E320 my alternator was going out and I only noticed it when I got into heavy traffic with alot of idling around. Same symptoms - radio turned off, etc and I think it went into "limp" mode because of lack of acceleration. When out on the open highway at 55-60 there were no problems.
Replaced the alternator and everything is fine after that.