1994 Firebird Shaking

I have a '94 firebird that has started running rough. I replaced all of the plugs and plug wires. I also replaced the EGR valve. However, when driving, the car idles rough and when on the highway when it shifts down to overdrive the whole car starts shaking when the gas pedal is pressed. Once you give it enough gas to get out ov overdrive it stops shaking. If anyone has any ideas please let me know. TIA