This is a repost from an earlier question - I've got more info and no real direction to go so I'm back.

This is a 2.9L I-6 non-turbo.
Car started showing symptoms of a bad fuel pump. Ran perfect most of the time then occasionally would stumble when given too much throttle. Problem worsened till now the car is unusable. Although it will usually idle and rev good as long as I don't push the gas-pedal too far. Sometimes it'll stall even if idling.
What I have done:
Changed Fuel Filter.
Checked OBD codes. The only hard fault has been an air-injection pump fault. It's been getting thrown for months before this problem appeared. Dunno if it's related.
When I monitor "live-data" the fuel system shows an O2 Sensor Fault. This isn't reflected with a "Check engine" code. Just live data. Dunno if that can cause this type of problem.
I checked the fuel pressure and it's showing a steady 42psi at idle and that increases to near-50 psi with the throttle. Doesn't appear to be pump/regulator related(or does it?)
I bought a new Mass Air Flow Sensor and installed it. No improvement.

The odd thing about this problem is that for a while - it would run perfect and have "spells" of this problem. The the problem gradually got more frequent till it actually stranded me. When I went back to get the car it started and I was able to limp it home.
When I first start the car cold - it "usually" will rev and take throttle fine but it seems to get worse as the temp gets to normal.
When I put the new fuel filter on it - it drove fine for a few miles then returned to the same problem.

Can anyone offer some direction? I could take it to a shop I know - but the parts are incredibly expensive if you have to pay full pop. The MAF sensor I bought was only $80 for a new one off ebay. I'd have paid $500+ for it at a shop.