Walrus, thanks for the information. By the way, I forgot to mention that I have a 1984 380 SL. I also found, like you, that thumping the dash makes the gauges come back down. My symptoms are very similar to yours except for the radio. I started looking closer into this when I was troubleshooting a cooling problem and noted that the gauge was inaccurate. Next, when troubleshooting a small intermittment miss at idle, I began to wonder if the idle computer/lambda processor wasn't taking a little hit here and there electrically, causing the fuel mixture to reset.

I'm diving in there today full-force with the exception of taking-out the steering wheel because, as mentioned in my previous post, I see some risks there based on previous work done in the car and/or how Mercedes put that steering wheel on. At this point the problem isn't bad enough for me to go that route though if I have to, I'll support the wheel (see my follow-up) and not rely on the wheel lock because I'm sure it'll break based on the torque (likely I should say) and will go to a garage with a power tool. Today I'm going to look at the wiring around the fuse box and passenger side, look for shorts to ground, troubleshoot the trunk light not working which may be a clue and basically trace-out what I can. I'll let you know if I make any progress and learn anything.