05 A6 AC issue

[dgraber460 at aol.com]

AC works great most of the time, but occasionally will start blowing hot air on one side or the other. No amount of changes or button pushing or turning the HVAC on and off stops the hot air. Vag-Com shows no codes in any systems.
Has anyone else encountered same or know how to correct this?
Patient is an 05 A6 3.2
The car also occasionally throws a passenger rear taillight brake light idiot light and in fact the light doesn't work but comes on later and is _not_ burned out. I have had the taillight out and cleaned,checked, and renewed the contacts 3 times. No joy. What the......
Love the car but this is taking a little of the enjoyment out of it.

Dennis 
Denver