Almost guaranteed it's one or more failed pneumatic servo actuators. There are three of them on the left side of the main air distribution box in the centre console. With vacuum shunted through the heater control panel switch, they open and close the vent shutters. It's the internal diaphragm that fails. The inability to hold vacuum in one servo can affect the others. It's an extremely common problem, but not an easy repair, especially as the Volvo parts are no longer available. The middle one has two vacuum lines to it and it's the one that seems to most often fail, leastwise it's the one that most often causes noticeable problems. There is a relatively simple bypass you can do blocking off the lines to restore partial functionality. Once you've got the left console side panel off, it's usually fairly easy to figure which one is leaking by attaching a length of plastic hose to each nipple and applying suction by mouth. You can access some of the hose connections without removing the side panel. Most people just do the bypass and learn to live with reduced functionality. There's all kinds of good info here in the 700/900 FAQ on both the bypass and doing a proper repair with substitute parts. There have been additional posts in more recent years here in the RWD forum with more tips and suggestions of other substitute parts. You'll have to try searching for them.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now