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Rough idle on a 2005 Chevy Express 1500 4.3 V-6 on 2005 Chevrolet Express 1500

Problem with the Chevrolet Express 2005 Express.
Rough idle on a 2005 Chevy Express 1500 4.3 V-6. At cold start everything runs great until I shut it off and wait 10 minutes then when I restart it runs badly coughing and sputtering and almost stalls out. I can nurse it along for about 5 minutes and then it clears up and runs fine. I was getting a PO430 so I replaced the cat on bank 2 but that solved nothing except I no longer get the PO430 code. Now I am getting a PO300. The strangest thing is that it clears up after about 5 minutes and runs fine. However, the other day I left it idling for about 10 minutes and it started its misfire problem again but only worse. It took about 20 minutes to get it to clear up that time. I took it to a local mechanic shop and they are stumped. Thank you in advance for any suggestions!

Asked by for the 2005 Chevrolet Express 1500
Any code lite on? Autozone will read em for free....Chang air filter also
I am getting a PO300