You're going to need to do some proper testing.

Monitor both pump voltage and current draw, as close to the pump as possible. This will at least give some clues as to how hard the pump is working and whether the power supply is holding up during this problem.

And also while monitoring pressure as close to the reg as possible

or you could just change the reg...as it seems about the only major item you have not changed ?

Has the problem just happened suddenly, or what ?