1999 Chevy Silverado PO300 Random Misfire

Thanks for your time. My problem is the engine in my truck will start up just fine but within 1 minute or so it will began to misfire badly. Right bank 2-4-6-8 misfire but 1-3-5-7 never misfire. I can put the truck in drive and depress the gas pedal 1/2 of the way down and it will misfire for a short time and then the engine will clear and run just fine. I switched all the coils on one bank to the other and same results. Before this started I had tuned the engine about 5000 miles before. I have new spark plugs, wires and new fuel pump pressure regulator. Some say it could be the cats are going bad but the muffler shop said they are ok. This is a 5.3 liter V8 Vortec engine. I get code 300 which seems to be a catch can code and no one seems to know what to do next.

Thanks