I have a 89 IROC. I recently changed some of the suspension. I installed the following UMI parts: crossmember mounted adjustable torque arm kit (the one with the shackels), adjustable panhard rod, eibach sportlines, Kyb shocks and struts, and a poly transmission mount. I have a horrible vibration, only when the car downshifts into 3rd gear. Car had zero issues with the stock parts, and now sound like the car is falling apart when I hit a small bump. Please help.

 

I have read the poly trans mount cause the vibration - in that it doesn't give like the original rubber mount. I started to put in poly and read about the issue and went with a rubber mount. I have had no vibration issues.