Hello, Jake here

New to the forum and was wondering if anyone had information as to this problem? I have a 2002 express 2500 5.7 and just installed a new motor and trans. The fuel inj.system under the upper intake, cap, rotor, plugs and wires are new and all sensors are old but were tested and are reading normal. When the engine was started it ran great for a hour then started to misfire. It is misfiring primarily in 3 and 6 cly. Did all the necessary checks with ignition and fuel and found the fuel pressure would jump to 60 then down to 50psi with the key on and at idle the needle would fluctuate rapidly between 48-50psi then with it off would drop 46 and bleed down to 32psi in 5 min. The fuel gauge on the instrument cluster also vibrates rapidly.