Does this ever happen when the heat or A/C is not on? AND when it does happen does the system go to full defrost? If it goes to full defrost then you are experiencing a complete vacuum failure in you climate control system. Carefully inspect all vacuum supply lines, check valves and hoses.
HTH

__________________
Scott Diener
89 300E
93 300E
92 Volvo 740 Wagon aka "Mutt mover"