Update: The dealership called and said they "think" it is the valve body, so they ordered one and will install it. I did contact GM customer asistance and voiced my concerns with 1) My lack of faith in their ability to fix the problem without "guessing" as they apparently don't really know what is causing the problem so they are just throwing parts at it. They already replaced a bunch of parts and the problem persists. If they completely understand how the transmission works, then they should be able to accurately diagnose it right the first time, and if they don't know how it works, they shouldn't be "repairing" them and 2) the lack of warrantee of the transmission when (if)it finally does get fixed. They called the dealership and reported that they are confident this will correct the problem, and GM then said they will wait and see before offering me any other kind of entended warrantee.