Cliff notes version first for those who hate long post. Over several days I have had intermittent fuel pressure loss, just to return to normal pressure the following morning. Filter changed and new lift pump installed. Overflow verified not stuck. Problem still there. Whats up?

Detailed version below.
On the way home from work last Thursday the fuel pressure began to fall as I went up a hill. Eventually fell off to zero. Arrived at home and drained a little fuel with the water drain at the filter , then cycled the lift pump. Repeat 2 or 3 times and got fuel pressure back. I then took it for a 25 mile round trip (all flat land)and just as I got near the house lost fuel pressure again. It slowly fell over a couple of miles until dropping to zero again. Early Friday morning I made the same trip and fuel pressure stayed above 10 psi. Drove a differnt vehihcle to work.

Saturday, I fired it up and it had good fuel pressure. Drove it for a 14 mile round trip and fuel pressure was low, but stayed above 10 PSI. It did set several hours before I retuned home.

Sunday afternoon came and I changed the fuel filter. It was very slow to build pressure through cycling the pump after the filter change.

So I swapped the lift pump out with a used one that I had replaced just before having the VP44 rebuilt and installing the fuel pressure gauge back in early 2005. It built pressure sooner so I was hopeful.

Monday came and I made the 35 mile trek to work with the pressure only maintaining around 12 psi. On the way home, coming up the same hill I lost pressure again. Stopped at Advance Auto and got a new pump. Installed it Monday night. I did a visual inspection of the lines from the tank to see if there were any worn spots before putting the pump in and saw no indications of a problem.

Tuesday came and I made it up the same hill without losing pressure down to zero but it did drop to about 10 PSI. Further down the road on a much smaller hill and in a 55 mph zone versus 75 mph of the larger hill I lost pressure again.

Cycle the pump once this morning and got only a few psi. Repeat and get 16 PSI. Went through an hour of cycling the pump, firing it up, repeat and it always hit 16 psi. Drove it to work again with about 13 psi maintained, dropping to 10 psi on the hills.

Driving home I mashed the hammer down pretty far in fourth gear on flat ground and the pressure dropped to about 7 psi and would not recover. Over the next five miles or so it went to zero again.

Stopped beside the road and did the filter drain and pump cycling routine a few times and got it back up to 6 PSI. Over the next twenty miles it slowly faded to zero again. When I turned on the 1/2 mile long dirt road to my house I hammered down on it once I got in third gear and into fourth. It pulled strong with no sputtering and the psi went from zero to about 3 psi.

Shut it off at the house and cycled the pump several times. Nothing. I am unsure if the pump was running with those cycles. This is a bump the starter for the 25 second run time cycle I am speaking of. Go out about 2 hours later to put it in the barn for further diagnostics and get 16 psi when I fire it up. Pulled the overflow valve out and checked it with air pressure to see if it was opening and closing. Yep. I don't have a pressure regulator on the compressor right now so I can't say just what pressure it was opening at but I could see the piston moving. I know I was getting air through even after I let the compressor bleed down to under 10 psi.

I am going to try to get a new overflow valve tomorrow. This is driving me crazy! Any ideas?