What makes my 95 Chevy 2500 with a 350 engine miss only sometimes at speeds starting around 50 miles per hours even though I have changed plugs wires and fuel filter?

Next things to check would be to make sure the gap on those plugs is correct, ignition timing, a crack in the distributor cap worn rotor or bad plug wires, faulty emmissions components, weak ignition coil, vacuum leak at the intake, dirty fuel injectors, leaky egr valve.