p0300 issues from hell

ok heres my problem My 2001 silverado started out life as 6 banger but now has been converted to a 5.3L. The truck keeps giving me a code for random missfire when on the highway or wot, when i check to see what cylinders it is it shows 100s of misses on each cyclinder. The thing is my 6 banger kept giving me this code before i replaced it with the 5.3l. Im lost at where to go Ive changed the fuel pump, o2s, stock mass airflow to a grantelli mfs and high flow cats. Both motors had headers, cold air intake, free flow exhaust and nelson tuned computers. ANy help or ideas would be great. Thanks

A code P0300 may mean that one or more of the following has happened:

Faulty spark plugs or wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Stuck/blocked EGR valve / passages
Faulty camshaft position sensor
Defective computer
Possible Solutions
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

can you feel the misfires during a brake stall or 4th gear, tcc lockup, accelerating?

did u put the stock metal boot covers on the plug wires? take em off. they may be arcing out

I did not use the stock boot covers because they didnt fit on the new msd wires. The plugs are E3 brand new. it only seems to do it during hard acceleration or cruzin on the highway the truck will go into a shutter and then ses light flashes and shows a code of p0300, i never have a problem with it while in town. Whats driving me insane is the fact that i had this exact problem with the 6 cyclinder motor so i upgrade the truck to a 42000 mile 5.3l and this problem is still present im lost on what to do. AS for feeling a mis during brake stall, 4 gear, tcc lockup Im not sure what these are and what to be looking for. Im greatful for the help! Thanks

Did you use the junk E3 plugs in the 4.3 too?

the truck has brand new plugs, wire, magnaflow cats, cold air intake, new o2 sensors, new masair flow sensor, new headers, new fuel pump, new injectors (because the originals had varnished closed from the old fuel while sitting in the junkyard), new pressure regulator. Whats wierd is that it doesnt always do it It some how can happen and then go away in the same drive cycle. AHHH!

no the 4.3 had accel and ac delco before I gave up and went to the v8. Are the e3s bad plugs?

E3's crappy plug. Stick with Iridiums. Either AC Delco or NGK's.

Ngk tr55