2005 TL Climate Control Problem

My 2005 TL's climate control seems to get stuck in "cool" only. Example: Temperature outside is 60 degrees. I set the CC temp to 68
degrees. I only get cold air (car is warmed up). If I force the temp setting all the way up to HI and wait a few seconds, warm air starts to flow (hot if I leave it there.). I reset the control to 68 degrees and from then on all is OK.
Anybody got any idea why this is happening? My thoughts were either a vacuum leak, a sticking venilation door or both.
Likely a dealer problem, but before I go spend the money, thought I'd see if there is a quick fix to this.
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