Is this after the turbo engages? My 740 had an issue with the blower shutting off under boost. There is a vacuum reservoir to store vacuum for the blower doors and it was not holding.

In my case there was a cap on the vacuum reservoir by the front bumper that split, not letting it hold vacuum.
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Paul NW Indiana '90 745 turbo 145 K

This appears to be a real gremlin in the 960 series. It's a vacuum leak. My son's 92 965 is also suffering from the same problem. Apparently there are two sources: one less likely is under the hood, and easy to fix; the other is under the console and may not be worth getting into. There should be air plowing from the footwells instead, so its not that your losing A/C.it's getting redirected to the floor. See if this is the case in your car.
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John Shatzer, '97 V90 @ 98K

I am having the same problem and I have to work on it. There are two easy to access check valves and one tougher one, I think, but check this post. http://www.brickboard.com/RWD/index.htm?id=672901&show_all=1
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Tom F '95-945na, '94-944na, lien holder on a '95-944Ti, RIP '78 244

Hi bru, hope this isn't a double post -- anyway my old Dodge did this regularly as a factory mis-design in the vacuum/diaphragm system that regulates the vent positions. Vacuum changes under load can suck the A/C vent door shut if there's a defect in the regulating system. A place to look. -- Larry C.