Bucking problems

I drive a 1999 Monte Z34, and when I am at 45+ mph, and go to give it a bit of gas, the car bucks. It's progressively getting worse. I have replaced the plugs and the wires, the coil packs tested out ok at the dealer, as well as the fuel pressure. Anyone have any ideas? Is there a filter I need to replace? Is the dealer giving me the run around? Anything please...