95 pontaic Bonneville hesitation

I have a 95 Pontaic Bonneville with 190000 miles and until now ran great with little problems. Now after the car warms up I have a hesitation when accelerating the car and cannot drive on the highway because at speed it bucks too much. I have replaced plugs, wires and fuel filter, checked firing of the plugs, additive to gas, changed tramsmission fluid and looked fine, and still have this problem. Any help would be greatly appreciated. The car also recently failed emmision with slightly high hydrocarbons, but that was before repairs were made with plugs and stuff.