First off where is your regulator, it should be as close to the carb as possible and it sounds like it's crapping out, bad spring. Start off by replacing it. IMO
I'M running 3/8" lines, a high volume mechanical fuel pump, filter just before the reg., reg. about a foot from carb, on a big block and no problems, press. holds at 6.5#'s hot/cold it don't care..