Subaru :: 2006 - Fixing CEL Code P0304?

2006 Subaru Forester X 2.5L AT non-turbo

I changed the oil and oil filter; replaced the air filter (Fram), and replaced the alternator and a/c belts. I also got a full tank of gas at Sam's Club, I was nearly empty. I've gotten gas plenty of times there, before.

Next day, the CEL comes on. It's a P0304 code. (Cruise control light is flashing, too.) Engine stumbles and runs rough when cold; idles fine when warm, runs OK, but not the same, when warm. CEL is steady, which is "not bad" but I can make it flash if I hit the accelerator while the engine is cold. Resetting the computer by disconnecting the hot battery terminal didn't work; CEL came on again two days later and symptoms remained.

I buy some MAF cleaner (CRC) and use it; MAF was pretty dirty on the upstream side, actually.

I'm going to change the plugs and wires (they need it, anyway) and I've ordered a new ignition coil, which I'll also switch out.

I've also ordered the gasket for the throttle body, which I'll take apart and clean once that comes in. Last step is to order replacement O2 sensors.

I'm 99% certain there's nothing wrong with the engine, in terms of rings, valves, pistons, etc. I think I'm covering all the bases in addressing this myself. I'm on a limited budget so I'm doing all the labor myself and ordering non-OEM parts on-line, after researching to make sure the parts have a good reputations, fit, etc. (Yes, that's all a big FU to Subaru service for being over-priced but besides being relatively poor I also enjoy doing this stuff.)