2003 Tahoe 5.3L stalling/lack of fuel???

Any help on this situation would be greatly appreciated. The truck demonstrated a hard start/ no start problem. After tapping on the gas tank the truck started. This lead me to believe the fuel pump/sender was bad. Brought it in & changed the pump. Truck now started OK. 1 day later truck stalled at highway speeds of 40-45 mph & at lower speeds around town. Brought into shop again, checked connections at pump-OK. Next checked fuel pressure-had 44 psi @ idle with fuel pressure regulator hook up. I read that pessure should be 54 psi. We disconnected the regulator & pressure went up to 54 psi @ idle. No fuel was noticed in the vacuum line for reg. Took truck for a drive with vacuum gauge attached & regulator disconnected. Truck ran fine & under acceleration vacuum dropped to near zero. What should vacuum be under a load & @ idle? Could this be a vacuum issue?
Thanks

Update-
Replaced FPR & all seems well, so far.