1996 Volvo 960, 2.9 Non-Turbo I-6 w/180K miles.
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 over a couple of weeks till now the car is unusable. The odd thing about this problem is that for a while - it would run perfect and have "spells" of this problem.
If I start it cold - it will idle and drive good until it gets to full operating temp. Then it will stumble and stall. It’s real good at making me think it’s fixed until I take it for a test drive. Then it stalls and wont start leaving me sitting on the side of the road. If I sit there until it cools – I’m able to start and drive it back home.

What I have done:
Changed Fuel Filter, New Mass Air Flow Sensor, New Throttle Position Sensor, Checked for vacuum leaks – found and fixed one. I checked the fuel pressure and it's showing a steady 42psi at idle and that increases to near-50 psi with the throttle. If I check it as it is stalling – the needle jerks between 40-50 psi as it’s stuttering. I used spray to check for intake leaks – none found.
R/R whole intake up to the actual intake manifold, inspected for leaks/damage and reinstalled confirming all clamps and plugs were ok.

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.

Other notes:
I pulled the coil-cover and noticed that at some point someone has replaced one of the coils as it looks newer than the rest.

Some of the suggestions I have gotten on this include: clogged fuel injectors, Clogged catalectic converter, bad coils, clogged fuel pump sock.
Most of those don’t make sense to me because they would be evident when the car is cold as well as hot – the car runs fine till it’s warmed up.
I have read lots of problems like this have been traced back to coils that malfunction when hot...This car has 6 coils and the reason I doubt that is my problem is simply that the odds of ALL of the coils or even 3-4 of them failing at the same time would seem like a real long shot.

Is there some part common to ALL the coils that could cause this?

Some background on me and my situation: My wife and I are short of cash in the short term and sold a newer car to get this one so we could lose the payment. We have a 2006 Honda and have been using that to get by - so this car has been “down” for almost 3 months now as I’ve bought parts each payday and tried to make it work.
I know it would be easy to take it to the stealer and get it back in a few days running like a top and I don’t mind paying labor costs but the parts for this car could break me. I have done all I have done so far while spending less than I would have paid to get the car towed to the shop let alone fixed.
I’ve got the tools and experience to swap any part on this car but I just can’t figure out what part it is.

warm? That has got to be the key. It runs like a new car until it’s all full operating temp.

 

Every car/motorcycle I've ever driven behaved exactly like that before the fuel pump went out. I think it's safe to say that's what it is.