I am having a vibration throughout the whole car when I shift into reverse or drive. It only occurs when I am holding the brake and sitting still in either of those two gears. In neutral or park the motor idles perfectly and does not vibrate, only when I am holding the brake in R or D. I replaced the rubber transmission mount, but that only decreased it a little bit. I was then told it might be a plug or my plug wires breaking down under torque. I replaced the plugs and that didn't do any good. I have yet to replace the wires, but they are only about 5 years old. Any advice would be appreciated. BTW: 66 Mustang coupe, 200 inline six, auto trans