[Quickening]

I can tell you, that buying premium gas does not gurantee not getting "bad gas". I have to use premium fuel with my turbo'd engine and I have gotten bad gas about three times.

I would do as others suggested. Have Onstar run a test or swing by the dealership and have an engineer come along for a ride. You drive it and when it happens point it out to them. It avoids the whole "could not repeat problem" BS.