I'd be wondering about the pump...they can do that, work intermittently, seem to deliver good volume unloaded but fall on their face under pressure.
The quickest way your gonna figure this out is with a fuel pressure gauge, any other way will just be guessing...you MAY be able to do the rent-a-tool from AZ or another parts store...
You'd wanna see somewhere in the 36-42 psi range at the rail I think...can't remember spec on these but that seems right. They'll run all the way down to only 10-12 psi I've found, but not well and starve out and die after a short time.
If it needs a pump, I'd strongly recommend getting a GM pump...I've seen a lotta problems lately with Airtex, what most places sell, especially on older GM's like these...don't know what the problem is, but I'm sure not gonna do that job twice if I don't have to.
Normal fuel hose will NOT withstand injection pump pressure for long. If you still have that questionable patch you might wanna re-do it. If you need to replace a section of the steel line, you can get what's called a compression fitting that will form a union between two sections of metal line...very easy to use. You'll need a small tubing cutter if you're not already using one, needs a clean cut. The comp fitting will have an outer fitting, and a little metal "bead" that goes over the line ends, then screw the fitting together TIGHT. The little bead will form a leak-proof union.
There's an o-ring suppose to be on the end of that line where it goes into the filter. Did you have it? They'll leak without it.
If you have to bend a new piece of line, wrap it around something round in a vice like a junk pulley or whatever...helps keep it from kinking as it bends.
If you can't do a comp fitting, next option would be to rent a line flaring tool (or sometimes you can find one cheap, I paid $9 for mine), put a flare on the lines where you patched, and use a high burst strength hydraulic hose the correct size. Then double clamp the ends of the hose, screws facing in opposite directions on opposite sides of the hose end. That should be a lasting repair.
But my feeling is that this person's fuel pump went out, and all the rest of this is just distraction. You need to verify what kind of pressure it's delivering when it cuts out.