Gotts

I have a 2006 A4 cabriolet that has an intermittent air conditioning problem. The problem occurs when I am driving at highway speeds for more than 2.5 hours. At that point the air conditioning cuts out but if I stop it comes back on for awhile then stops. I brought the car to Audi and they ran it for a few hours but could find nothing wrong. Driving around town and short trips does not affect the normal functioning of the AC. Has anyone come across this problem and how do you troubleshoot it?

Gotts