1989 4.3L engine miss

1989 C1500 4.3L with 90000 miles. This problem is a little hard to define. While driving at highway speed if I let up on the gas just a little the engine will start to miss/buck slightly. If I unplug the egr valve the problem seems to disappear. Engine idles well.

I have replaced the distributor, coolant sensor, tps, idle control, egr valve and solenoid, map sensor and oxygen sensor.