Falcon bad vibrations

I own a '98 EL XR6 Falcon. There is a vibration coming from the motor when idling that transmits into the cabin. It feels like the car slightly loses power then gains it again, but the rev counter doesn't show any signs of this. When you look at the motor you can see it is shaking. All the leads and spark plugs have been changed. I took it to the Ford dealer and they said the idle speed control unit needed changing. It got changed but the car is still doing what it did before it went in. How can I fix this problem?