Volvo: 98 S90 - air/heat control mixer problem, vacuum lines, control mixer

Question
Ray, I have a problem with my '98 S90 that's driving me crazy.  Every now and then the air conditioner (or heat, depending on the season) air flow just stops.  Seems not to matter if the "return" is on, or on outside air.  The fan is working - I can hear it....just no air.  I understand the control box is activated by vacuum.  I've had it in to a Volvo mechanic and when he checked it all was well and he couldn't see the problem.  It's back to "No Air" now, and I'm very frustrated.  Is there anything I can do for a simple fix, or any fix for that matter?  Thanks in advance for your help.  Jim

Answer
Jim,
So right now no air flow. Look for vacuum at the lines that go into the car. Right side by the A/C pipes. If there is vacuum there and still no air flow go inside to trace the vacuum lines to find where the vacuum stops. Does this happen when you accelerate only? If yes test for a leaking vacuum bellows left side of the climate case especially the one that has two vacuum lines to it.