Like Maxwedge mentioned, bad injectors will cause what you describe. At the speeds in which it's happening, it could be a TCC lockup issue(as that is the TCC lock-up threshold), but if it's that bad, the stealership should have been able to pull a code on it. Bad injectors don't trip a code of their own, but will sometimes cause a random misfire or specific cylinder misfire code if only one injector is bad. I also agree that you should talk to the stealership service manager, as I would be insanely ticked off after spending $800 and my car was not fixed.