This is a common problem on the 960's. You either have a vacuum leak, bad check valve or a bad vacuum servo in your HVAC system. Check the vacuum lines going from the engine into the firewall on the passengers side for leaks. Check the 2 check valves for proper operation. See if you can here any vacuum leaks behind the dash. I find this is easiest done with the engine off and a hand vacuum pump.

Frequently the dual acting (dash/floor) servo is the culprit. When the engine is running with a light load, it is producing enough vacuum to keep the bellows working (vacuum leak in the rubber seal), as soon as you gas it and the engine vacuum goes way down, the residual vacuum in the reservoir is bled down and the bellows goes into a neutral mode. That's because the bad bellows is a dual acting servo. The fan doesn't slow down but all the air comes out of the floor vents. As soon as you let off the gas, there is sufficient vacuum to overcome the leak and the bellow moves back to the dash position.

You can temporarily fix this by putting a screw or a golf tee in the yellow vacuum line (after taking it off the dual acting servo) on the drivers side of the HVAC unit. This can also verify if the leak is in the servo, rather than somewhere else.

There are instructions in the FAQ's detailing how to fix it.

DEWFPO
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1998 S90 083,228 and 1995 964 154,100