Check that the ht leads are separated by lead bridges. If the ht leads are not of high quality then touching leads allows for an induced current which fires across a plug where the compression is low. High compression as in firing stroke needs an increase in voltage to jump and if the pressure is less in another cylinder then the spark will go to there.
Testimonial: "thanks for your response. I'll take another look at the clearance. The 05 has individual coil packs with about 4-5 inches of separation."
ok so it will not be the HT leads. sandi rossi has a very good point as the cylinders misfiring or opposite banks at the front and back. When fitting a inlet manifold these are the areas that are most susceptible to fitting or gasket failure. Run with that suggestion and take extra care to tension the manifold bolts in the correct order and to the correct tension
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Sounds like you have checked a lot of things if it was mine I would do new intake gaskets, I had a 2002 Sonoma I did a lot of what you did and every time I gave it back to owner a few days later lite was on and p0300 I did intake gaskets and its been good 2 yrs now,not sure what gaskets you got but gm use plastic ones I got new gm ones and they were aluminum type call gm dealer parts good luck
Testimonial: "thanks for your response and I will take another look at intake gasket."
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