Toyota Highland Temperature Control

I have a 2005 Toyota Highland V6 3.0. The car runs great, since last year, when the AC was on, Sometimes it suddenly blew out the warm air, and it had to turn the temperature knob around, back to cold position, and then cold air came out again. If someone had same experience, can you share how you fix it, and what part needs to be replaced?

Thanks your help.


Michael