I double checked all the vacuum hoses and they are all in good condition
I received a replacement air and fuel filter today that I ordered along with the O2 sensor (separate shipment), and it did the trick. It's kind of embarrassing that such a simple thing was the cause, but it was only one piece of the puzzle. It was a combination of: blocked cold start injector, broken O2 sensor, and soiled filters. Thanks everyone for all the help and advice.