Rip out those gapless plugs. They are know to misfire. Replace with FACTORY plugs–AC Delco.
Next, clean the MAF or replace it. Then, check for vacuum leaks.

A lean condition means too much air is getting in and combustion can’t complete. So you get misfire codes. Since they’re not on a single cylinder (if it was you’d get a misfire code for that cylinder), you most likely have a fuel or air mixture problem. A lean condition would also cause low power and bad fuel economy.

GM MAF sensors have a high failure rate.

Also, get yourself a scan tool with live data and start reading the data stream to see what’s going on. You’ve already wasted more than the cost of the scan tool in wasted parts.