I had the same issues you described for about two years with multiple trips to the dealership and they indicated that they would have to start changing modules on the communication buss. I bought a good scanner, a subscrition to Alldata and started chasing. It would act up at temperatures above 85F originally but got bad toward the end at 75F. I changed batteries and checked grounds, everything suggested on the forums.
I finally fixed the problem by cleaning the connector on the anti-lock breaks with a spray can of contact cleaner from Radio Shack and some small wire burshes. 18 months later with several trips above 100F and no issues.