Do you have Electronic Climate Control (ECC)? If so, does the display flash?
I would pull the glove box out (just a handful of screws - real easy), grab a flashlight, and watch the airmix actuator in operation. Run the temp back & forth from hot to cold. The white door arm should move smoothly - no jitters, hesitation, etc. - and should move to the same positions each time at max and min. Any discrepancy there would indicate a suspect actuator. Pictures 17 & 18 in this http://trialsnuts.com/ecc.pdf
may help you determine where to look to watch the action. The article is long (I know the hosehead that wrote it
), and much of it won't be applicable to you, but there may be some information that will help.
Anyway, I'd do a visual on the airmix actuator first because it'* easy and doesn't cost anything (as the resident tightwad, I would know).