01 red pony

I have an '01 GT vert that is acting strangely. When I have my top up and i'm running around 60-65 mph,the whole car gets the shakes. I'm talking the steering wheel shakes, my hood even shakes,you can even see the bad vibes in my drink thats in the cupholder! But here's what gets me, when the top is down their is hardly none if any vibrations or shaking going on! The wheels have been balanced and it's not out of line. If anyone knows of anything to do for this or what could be causing this please drop me a line and let me know,because this is going to drive me crazy! It will be greatly appreciated.