I know it’s not a Mustang but still the same engine. I am working on my sons 1994 Cougar XR7 3.8 V6. It has about 35,000 miles on a dealer installed remanufactured engine. This engine just started to vibrate badly from just over idle to about 1,500 rpms, after that the vibration disappears. It starts well and runs strong without missing. It vibrates at this rpm whether on the road or if I brake torque it in the driveway. I suspect the harmonic balancer has failed, as it is the original from the first engine that was in the car. That engine had 120,000 miles on it before the head gasket failed. Is this usually what happens when a vibration dampener fails? I have never had one fail before. Thanks!