Chris, my 1993 960 2.9 has similar issues as yours but only on one of the servos...and I live with it knowing it will go warm for a few seconds when the manifold vacuum reduces, but it soon comes back when you lift your foot off.
There is a full description of how to fix these servos (i.e. direct replacement) but as they are no longer available you can "fix" the old ones. The description is in the FAQ on this site under "HeatingVacuumServoReplacement.htm#VacuumServoRepl acement" and it has very good pictures of the process on a 960. To repair an old servo, you need to get access as described in the FAQ and repair the crack/leak in the rubber of the diaphragm with a little rubber repair sealant or similar. It does work 'cos I've done something similar on another vehicle, but work carefully from the procedure described. Its reasonably easy to get access on the 960.
Have fun!
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