1997 Chevy Pickup  4.3 Vortec v6 service engine soon.
OBDII Codes = P0300
Work done prior = customer installed intake manifold gaskets.
Done to repair = new plugs, wires, cap, rotor, cam sensor, crank sensor, mass air flow sensor, fuel
filter, fuel press regulator.

Question = i need to know what i could be missing or need to ck next this truck has good
compression good fuel pressure it will run fine for a while you can drive it for 8 miles then it backfires
and starts misfiring. I'm at my last wits on this one any help will be great full.

Well, really need to know what cylinder(s) are missing first so to concentrate on those. Some  things
could be- sticking injectors after they warm up, carbon floating around in the egr  valve/passage,
possibly timing is off.  Check to see if the ignition coil is breaking down when hot  by having someone
brake torque it and spray some water around coil. Curious as to why cam,  crank, MAF sensor was
replaced. Was there codes for those?

in regards to the last info i gave you this truck shows to be miss firing on all six cylinders 5 and 2
show to be missing allot more. I slotted the distributer hold down to try to adjust cam retard to 0 and it
still misfires on all six cylinders has a good vacuum reading and i replaced all the injector poppets
with a known good one please help.

With all cylinder misfiring, 2 of them more than the others suggest a plugged
catalytic converter or  
poor fuel quality. Did the customer install the lower intake gaskets properly? Maybe the wrong  
gaskets. I would remove a couple of the plugs and inspect them. Make sure not burning coolant.
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Chevy 4.2L Vortec Code P0300- Engine Misfire