My 2002 chevy venture is misfireing, had codes read at autozone and code was misfire on #6 cylinder

I replaced lifers,pushrods,head gaskets,lower intake manifold gaskets,upper manifold gaskets,spark plugs,wires,airfilter,colant change,oil change, some times when im driving down road the check engine light will blink some times and it only idels rough but when i start to drive it smoothes out and runs fine