Firebird shakes when I hit the brakes occationally

I have a 1985 Pontiac Firebird. Not always but occationally when I hit the brakes the car goes into a violent shake. This shake moves the entire car and there is nothing I can do to stop it unless I come to a complete stop. Any ideas of what to do?

the shoes and the pads are all brand new and the rotors and the drums were all checked and are fine. The brake lines are all new and there is no leakage