fuel pressure dropping over time

been able to take the duster on some short trips around town, and ive noticed something annoying...after a while the fuel pressure drops


starting at the back there is a new (so clean socked) 3/8 fuel sending unit
running through a 3/8 fuel line to a carter M6270 mechanical pump
then into a Malory 4309 return style regulator (using the stock 5/16 line as a return)

when i fire it up, it reads 6-7 PSI on an autometer electronic fuel pressure gauge (looks like a Z-series, but not sure)


after i drive for a few miles and everything gets up to temp, it drops down to 3 psi, at which point it seems to struggle keeping the carb filled (if i let the pedal down to far, that is)

last time i put the car away, it was reading 4 PSI, and removing the gas cap did not register a change, so i think we can rule out a bad tank vent causing a vacuum



any other things to look for?