I think that is in the year range that the air conditioning controller to the left side of the glove box can give problems. The system is not as straight forward as it seems to diagnose. Have someone that specializes in Lexus repairs, if they get stumped there is a professional service called Identafix. http://www.identifix.com/info/about_us.htm They only deal with professional repair shops. They are in the Minnesota 651-633-8007. Your auto repair shop do all their diagnosis tests, and discuss your problem with a team of professional technicians great service cheap and very bright people. If the repair shop that are repairing your car are stumped this is a great service. This company only deal with automotive repair shops and don't answer questions for the general public but are an amazing knowledge base.
Sounds like air flow problem at idle, check the a/c fan(s) for problems. Also see that the refrigerant level is correct and full. Requires special equipment for refrig recovery if needed so recommend a qualified shop look at it.