OK... curiouser and curiouser...
I used VAG-COM to convince the radio that it has an attached CD changer by 'recoding.' I could not convince the HU that it should be a monsoon... it refused to accept that bit.
After a few days I got a CD-X again and immediately plugged in VAG-COM. The radio had a stored code:
"00857 - CD Changer Unit (R41)"
" 49-00 No Communications"
I power cycled the PB and turned the HU back on, and then drove to work listening to the PB most of the way. With the PB still playing, I killed the engine and switched back to run, then fired up VAG-COM again to find:
"00857 - CD Changer Unix (R41)"
" 49-10 - No Communications - Intermittent"
I had no means of reading or setting codes before acquiring VAG-COM, so I don't know whether these codes existed every time I experienced CD-X or if this level of diag only became active when I re-coded the HU.
This CD-X problem has been going on pretty much since day one, with my initial thoughts along the lines of a thermal problem (did not seem to happen in winter here in the northeast US, but happens lots in the summertime.) Venting the disk cart and adding a heatsink to the bottom of the PB in the area of the power supply FET did not resolve...
Unplugging the PB so many times led to a broken connector, and the poor quality of the VW OEM connector led me to replace that connection with Molex. The problem is still repeating.