The problem could be many things, but it could also be something simple. Now on the simple end, it could be your tires are all different. That happened to someone I knew. The minute he hit 60 on the expressway - his car would start to shake almost uncontrollably. We took it to a shop and they couldn't find anything wrong with it. But then we took it somewhere else and the guy was like "you know you have 4 different makes of tires on this car!" so we fixed it and the problem went away. It could also be that you need a good tune up of the system. If cars are bought and sold by a dealer then they have no responsibility to tune the cars up. As long as they run they can be sold. I would honestly find a good body shop in your area, let the guys know that you want to mod your car, put some new gear in and make sure to tip them well. As long as your honest about it, youll come out on top.