A roundup of the usual suspects would include the Air Mix Servo. It's one of three servos located under the dash at the center console: (1) Air Mix Servo, (2) Mode Servo, (3) Air Recirculation Servo. The Air Mix Servo controls the amount of hot air entering the system to attain/maintain the set temperature. I just replaced my Air Mix Servo for the same reason and while I was in the area I also replaced the Mode Servo. You will find a lot of discussion/instructions/pictures on replacing these servos here: Servo Replacement Info
. You will find discussions on how to disassemble and clean the servo contacts but that is only going to be a short-term fix. Your best bet is to just replace the Air Mix Servo. Even if the Mode Servo is working OK now, given the degree of difficulty to access these (actually, it's not THAT difficult) I would replace it while I was in the area.
I ordered my servos from Stevnison Lexus, Frederick, Colorado (Servo Parts Listing
). Their online price was a LOT less than what my local Lexus dealer quoted. I offered them the opportunity to match the price but they couldn't do it; they couldn't even get close!!
Lotsa luck. Hope this helps.