What do you think about the last line of post 7......fuel pressure regulator?
Have you tried swapping regulators with someone to see if the regulator is bad?

If you swap regulators and the pressure is still inconsistent, it may be the fuel pump.
A new fuel pump is no guarantee that it is a a good one and with a known good regulator
and.......... if all your filters are good, the pressure should get back up.

Next, check the gas. sometimes I have seen the gas/ethanol "separate" and it looks like
rusty water when the car sits for a while.-Bob