Misfiring and backfiring

Yes, the check engine light is on. I had Advance Auto run a scanner on it. I think it said number 3 cylinder was misfiring, o2 sensor and something else maybe but I don't remember. I haven't had a chance to check all of that yet but I have changed my distributor cap and rotor button, pick up module, wires, plugs. I am a shade tree mechanic don't know everything about an engine, but I can work on older models and rebuild the engines. But this 4.3 engine runs pretty good until it reaches its max temperature then it would start missing. But if the engine runs at 195 degrees at normal running, it would not run bad or misfire until it gets 192 degrees. I'm just not sure what it is. I don't always have inter net service. My name is Scott Smith please help me.. Replacing electrical parts can get pretty costly. Thank you for your time.