I have a 94 z28 m6. when the car is slowing down without the brakes applied there is a strong vibration in the floor and the shifter starts to move up and down really hard. I just changed the motor mounts and the trans mount and everything else feels tight. I can press the clutch in and it will still do it. Ive gotten it to do it while cruising at about 40 and above. I'm lost and it has never did it before so its driving me nuts. Any advice would be appreciated.

 

Did you go through the basics? Propshaft U-Joints, Suspension parts, wheels for balance and torque, torque Arm mounts. Even though your engine and trans mounts are new that doesn't mean something didn't cause them to go bad or loosen a bolt already. Is it more speed related? Hard to pin point things over the web.

Jeff