This is another one of those known S40/V50 quirks. The HVAC servos can get out of sync and it does exactly what you described. I had the same problem about a year ago. Unfortunately, all you can do is take it to Volvo service and have them do their magic (software reload / calibration). I believe that there is a software update that attempts to fix this issue.
Stan
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2005 V50 T5
IPD Rear Sway Bar
2002 S40 1.9T - R.I.P.
1998 V70 NA - Sold
Remember kids, Speed doesn't kill ... impact does!
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