Under the big eotech, plastic, top engine cover, On top of the valve cover, is a small black box. This containes the 4 spark plug coils that fire the plugs.
You had tomremove it to change your plugs.
Check GMParts, AZone, other local parts housed and RockAuto.
A poorly working coil pack will reak havock on your engine and at 130k mi, it is not out of line for PM for $20-$50 for a new one is cheap compared to a damaged Cat due to misfire.
I see you gapped your plugs, checked the spark plug gap....good
Did you clean the mass air flow sensor?