I had the codes read on my 2001 Silverado 4x4 5.3L vortec. It was having power problems. Came up with a p0300 and o2 sensor. I replaced 02 sensor. Still had the p0300. Changed the fuel filter and no help. Took to a local mechanic. He ran it on his diagnostic computer. 2,4,6 and 8 cylinders are randomly misfiring. He checked the compression, its good on all cylinders. Changed sparkplugs, wires, coil packs, EGR valve and intake gaskets and the plugs are not fouling out either. Ran it on the diagnostics computer and checked the computer and all sensors. Everything is working. Basically, he has thrown parts at it, and not been able to fix it. He wants to take the heads off now and check the valves and the Head gaskets. Anyone ever heard of this? Also, cylinders 1.3.5 and 7 are fine. No problems.