Began as a no start now low fuel pressure

I bought this van a few months ago. It worked great. Until a few weeks ago I got in to go to town and it would turn over fine but would not start. Looked under the hood and saw no problem. Got back in and it started right up and continued to do so for several weeks. Then one day I stopped at a store (on a slight incline) and when I came out same issue. A scan showed a p-code for misfire on number 6. When it would still not start the next day had it towed to a shop. They kept it a couple of weeks and it started every time. They did have to put in a new battery. I brought it home and since it was fairly new to me I replaced the air filter, fuel filter, spark plugs, COPs, inertia switch and discovered the fuel pressure that had been between 30 and 40 psi was now down to 18 to 20 PSI. I replaced the in-tank fuel pump; still low. I replaced the fuel pressure regulator; still low. Still starts right up every time and runs fine. Baffled, I took the new AC Delco fuel filter out and put the old Motorcraft filter back in and the pressure jumped right up to 40 psi. Must have been a bad new AC Delco filter. Bought a new Wix filter and put it in and the pressure is back to 18 to 20 PSI. It still starts every time (can hear the fuel pump prime up) and seems to run fine. When you turn the key on the pressure slowly climbs to about 12 and then slowly goes up to 20 when you start the engine. With the old filter soon as you turn the key on the gauge jumps up to 40 and when you start it drops down to about 33. I put the gauge on before I took the AC Delco filter out and never moved it off the schrader valve throughout all three filters, so I know the setup was the same and the gauge is good. What is going on?