I have a 98 silverado k1500 with 239000 miles that has a p0300 code. I have done a whole ignition tune up... new wire-set, plugs, distributor cap, ignition rotor, ignition coil. So basically it is like chasing a ghost. I might be thinking that it is low compression in a cylinder. But im just not sure... Also it has a 350 vortec 5.7L engine.

 

Might want to check that compression since you've checked everything else