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I sure would like to find out more on this too. Our '95 850 turbo wagon is doing what sounds like exactly the same thing. Started on a road trip with the family last week best I can tell. We had the system purged and recharged a few months ago by my neighbor (he is a certifed AC tech and evacuated the system and charged it by weight) when the 134a charge was low. AC has worked great since. What is happening on ours is after about 15-20 minutes of operation nearly no airflow comes out of any vents, no matter what position the knob is in. You can hear the fan laboring away, trying to push air through a blocked climate control system. We pulled off the road to check things out, since we were heading to the Sierras and passing through the Fresno, CA. area with no AC was really not going to be fun. I yanked the radio and Climate Control Head...wasn't able to see anything obvious. By the time we got going the CC system was blowing some cold air. Later on in the trip I discovered that if I turned off the AC manually that in about 3-4 minutes the system would return to normal. I also noticed while filling up the car and doing a pre-trip underhood check that one of the AC lines appeared to be much more frosted up than normal.
Here's my theory. The 134a charge is such that the evaporator is freezing/frosting up to the point that it is blocking air flow. Either that or whatever controls the CC air mixing box flaps is failing. (If it were an older Audi I would simply resolder all the vacuum solenoids in the programmer and call it a day--but my 850 knowledge, not to mention patiance with this car at this point, is lacking.)
Any 850 automatic climate control experts out there in Brickboard land who can help offer some troubleshooting advice?
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