I have a 2005 GMC Sierra with a 5.3. It starts and idles fine but as soon as you try to put it under load it just dies. It has new catalytic converters, new O2 sensors, and a second new fuel pump. Fuel pressure at ignition on and idle is 60 psi but as soon as you accelerate it drops to 20 psi or less. should note that this fuel system has the fuel pressure regulator with the pump. Also, no codes set. I am at a loss. any suggestions would be appreciated.