How to determine which fault code PO303 is coming from

Hello, I have a 1998 GMC Jimmy 4.3L. I just recently did a tunup which includes all 6 sparkplugs, 6 wires + 1 coil wire, distributor cap and rotor; fuel filter. Now I get a code PO303. I took to autozone they said " a misfire on cylinder #3 and or fuel injector faults"; they are other faults too I think may not be the problem. I remove sparkplug #3 and checked the gap and reinstalled. I checked sparkplug wire with ohm meter and got similar results compared with other wires. The check engine light comes on when car seems idled; but it goes off after awhile. My check engine light is off now. The car drives smooth with the exception from time to time it seems to stall then drives good. It does have a rubber burning smell that is not noticable until you have drove for my then 30 minutes or so. I think that is just smell from new wires put on and it will die down over time. Currently, someone suggested to try "fuel injector cleaner" which I am doing now. If this fails, what be the best next course of actions? Thanks in advance.