The car in question is a 94 850 with the manual climate control.
Two years ago, the air conditioning was working intermittantly due to a bad pressostat. It was replaced with genuine Volvo p'stat. Pressures all looked normal for R134.

This Fall the AC was acting up again. Sometimes a light rap on the pressostat would get it working other times not. Putting a jumper in place ALWAYS worked. We've been blessed with some warm weather over the past few days, and the AC refused to work. Even rapping the p'stat didn't fix it. Figuring that I do have the original evaporator, and they are known for leaking, I ASSumed that it was bad and that I was low on refrigerant. This afternoon, I popped the gauges on, and found the static pressure more than adequate to trip the pressostat. When I jumped the p'stat, working pressures were good. ie, I have a full charge and everything else is OK too.

So it seems to me that replacing a pressostat every two years is excessive. Is there anything that could cause early failure, or is it just bad luck?