2000 Tahoe problem child

[trixterrrr]

I have a 2000 Tahoe with a 5.7 vortec. I have changed head gaskets, intake gaskets, injector spider assemble, coil, complete distributor with cap and rotor. I checked for plugged exhaust, found excessive backpressure so unhooked the cats at the manifold, still have miss. The wire are MSD's which are a year old, checked continuity on the wires and swapped with different hole anyway. New plugs which I also swapped with a different hole. I also checked compression on all the hole, and there all good from 120 to 160 psi. And #3 has 150 psi of compression. I get code PO300 on the scanner, which it shows #3 missfiring and #5, however if I rev it up to 1500rpm #5 seems to hit. I put a spark plug tester inline on #3 and the light flash very dim and once in a while it will spark bright. I checked #2 with the tester and it was very bright flashing all the time. I am very confussed, thinking maybe the ECM, any ideas would sure help.


Thanks


P.S. Have new catalytic converters on the way.