when im driving my car "crusing" i can feel a big vibration(like up/down), at 1500rpm to 2500rpms, like just pressing the gas pedal slowly almost halfway, that you feel the car is picking up speed but reeeally slow just before it downshifts to a lower gear to pick up more speed. If i accelerate more so it downshifts, the vibration goes away. When parked and revving, vibration doesnt happen. any suggestion?