if balance and alignment work out, then look at the tires. When i bought my GT i put 4 brand new tires on it. 2 of them w/less than 500 miles had broken (defective) belts. This caused the same sort of feeling. Its tough to see visually unless you know what you are looking at. Run your hand around the middle of the contact surface on your tires. If you feel and bumps or dips, thats it. Good Luck!