rx8newbie

When driving long stretches and slow down or come to a complete stop then speed up again, when I hit 45-55 the car starts to shake like if the transmission is in a gear that is to high for the speed. Has this happened to anyone. I've reported the problem to my dealer and said he fixed it with a new TSB but it still does it. Again only on the long runs. I bought car new but I recently saw one on ebay that said it was a Manufacture Buyback that was corrected for this same issue. Is this common and what is Mazda doing about it?