bucking after 20 mi.

got a 1995 sable with 3.8 (90,000) and have a problem with the car bucking after about 20mi. or 1/2 hour driving. replaced plugs,dist.cap,plug wires,fuel filter and have tried several fuel injector cleaners. runs fine at first, then starts to buck,sometimes a little bit sometimes a lot. anyone had same problem?