Radio gremlins mystery almost solved!
 

I've had about 8 incidents in 4 years of the station changing slightly, or, rarely, the tone controls being messed up. I got back to Dallas from a week in Asia and started up the A6, and the tone controls were up all the way. Then I remembered the storms and the 8000 lightning strikes in Dallas county alone.

As far as I can remember, my radio incidents were co-incident with thunderstorms or lightning nearby. This makes perfect sense. There isn't enough EMI shielding in the radio and strong electric fields can change a few bits of the memory. Any EE's want to comment?

only one

I had that problem but a new radio solved it. gremlin free for 5 months.

And all it took was a new radio? ;-)

Nope, its a known software bug.
 

FWIW, I had this issue with my first Symphony radio, and had it replaced. An former coworker who had worked at Cosworth Engineering (for VW and Audi) told me about the bug even before my radio developed "symptoms".

These days it is EXTREMELY unlikely any EMI that the radio could be subject to in a normal environment could cause any damage to the audi system. I did EMI/RFI testing of some OEM automotive products that I designed, and it takes a hell of a lot (maybe if you aimed the radar from an F18 at it, or an EMP - OK, maybe it wouldn't take quite THAT much) to cause them to malfunction, unless there was some serious design flaw, which would have showed up in testing. On top of that, these issues would not always be the same, as the channel/tone setting issue is. External EMI or RFI would be much more likely to damage the front end of the tuner circuitry, not internal memory circuits.

My theory is that the internal memory writing routines that save the current settings aren't able to ensure valid data over the tolerances of the parts (extreme heat or cold and age of the components makes it worse). So, every once and a while only in some radios, the data does not get read or written properly and you see a glitch.