I'll try choking the return and retest. I should have known to do that, I've told people to do that before when checking the FPR. Duh. Thanks for reminding me.
I cleared the codes and drove to work, got a few more misfires but nothing like before, pulled hills well and only missed if I really hammered it.
I was kinda thinking back and it all started the next day after filling it with gas, and the temps were really cold that night. I'd managed to burn off 3/4 of a tank of fuel messing around trying to diagnose it, and threw some Techron in it when the trouble started, so I refilled it last night. Had a couple of hiccups when charging up the ramp onto the highway, less than 1/4 mile from the gas station, set PO304 and PO300 again...this morning it ran a little rough at first start after sitting out in the cold all night, but straightened out quickly. All the way to work and back it ran perfect, and set no new codes.
I suppose I coulda got a tank of shit gas, but it seems like even if so, it's trying to tell me something else that needs attention. I'll keep on it, and let ya know what I find...