That's it in a nutshell. I replaced the pump 2 years ago when it wouldnt start, filter a month ago. She starts with starting fluid and runs (lacking consistent power).

So it seems like intermittent fuel starvation. This happens at low and high speeds. With AC on... it dies.

Thanks for the comments.

That's it in a nutshell. I replaced the pump 2 years ago when it wouldnt start, filter a month ago. She starts with starting fluid and runs (lacking consistent power).

So it seems like intermittent fuel starvation. This happens at low and high speeds. With AC on... it dies.

Thanks for the comments.

What engine?
Tune up history?
How is the fuel filter now it should be rechecked if hard to start or fuel pressure and volume is down.?
What is the fuel pressure readings?

What engine?
Tune up history?
How is the fuel filter now it should be rechecked if hard to start or fuel pressure and volume is down.?
What is the fuel pressure readings?

4.2 Vortec V6
Filter... unknown
Pressure.. unknown

4.2 Vortec V6
Filter... unknown
Pressure.. unknown

Hard to start cold.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.

Instead of simply replacing the pump again as a guess, get your hands around (beg, borrow, buy) a fuel pressure gauge. Test standing pressure (KOEO) and pressure at idle. If the standing pressure is not 61 PSIG or higher, starting will be difficult, if not impossible.

Instead of automatically condemning the pump, pickup screen, fuel filter, gremlins, or whatever else you might blame, test the pump voltage as close to the pump as possible. There should be an electrical connector just ahead of the fuel tank along the left box frame rail. Backprobe the connector there and check voltage with the pump running. If you get less than 12.2 VDC, the problem is not likely the pump. Start looking at connections, the pump relay, and other wiring. If you get 12.2VDC or higher, that still doesn't condemn the pump. The FOUR electrical connections right at the top of the tank (yes, there are FOUR connectors - On the pigtail, on the tank top, just inside the tank top, and at the pump itself) are suspect, and are known failure points. Any resistive connections there can lower pump voltage enough to drop pressure to a useless level.
This is what usually happens:

http://72.19.213.157/files/PumpConnector.jpg

EDIT:

It figures - By the time I got my reply together, you already got your answer from a couple other members. MT2500 is all over it. The only thing I'd differ on is the minimum starting pressure. I know what the manual states, but I've seen a few of them start and run as low as 60 PSI starting and 56-57 PSIG running. That doesn't mean they'll all start that way, and it doesn't account for slight variations in gauge calibration. A good pump, with a good pickup filter, good inline fuel filter, correct FPR, and adequate voltage, should make almost 70 PSIG KOEO, however. Dead-headed (return line closed), it should hit 80 PSIG or more easily. Just don't run it in that mode any longer than necessary to get a good reading.

I phoned a GM Service person when trying to trouble shoot my '97. The rep said "minimum 60" psi. (for starting). I said I had 59 psi, (on a borrowed guage), the rep repeated "minimum 60".
The replacement pump I bought included a dielectric grease to fill that connector ontop of the tank / pump fitting, with warnings of checking that connector. I was surprised at the small size of the connector, considering the pump draws about 10 amps. Also, there's a black ground wire at the front of the tank, bolted to the frame. Mine was loose and corroded. (which I fixed before I changed the pump, and after I changed the fuel filter)

So you've established that the pressure is too low, but have you checked the running voltage at the pump (or as close as you possibly can)?