96 Chevy C1500 random misfire problems

[AFROBUSH]

I have the 305 v8 5L engine, and it seems to be misfiring. It only begins to misfire when i am going above 50mph or right before it changes gears. After i fixed the check engine light i scanned it and the random misfire code came up, and a code for a bad O2 sensor after the cat converter did also. If anyone knows what could be causing the misfires i would like to know. I have already changed all the spark plugs, and the wires, and the problem is not fixed.