300 code misfire on #4 cylinder 2002 chevy s10 pickup 4.3

Question

problems with check engine light flashing saying misfire on no.4 cylinder 2002 chevy s10 pick-up but there is no misfire it will flash and then go out

we have replaced dist .assy. crankshaft sensor, all lifters and spark plugs and wires and fuel injector assy. coil, cap & rotor, timing chain assy. intake and upper gasket sets and pcm still after it warms up the light starts flashing after 15 to 20 minutes am stumped