Engine cranks over but wont start

Replaced fuel filter and fuel pump, replaced fuel injectors, crank sensor, and fuel regulator. Engine will run if gas is poured into the throttle body opening but will not run on its own. Good compression (over 100 psi). Want's to run but it seems that the fuel supply is shuts off when it attempts to run. Fuel pressure is 40 lbs at the test point in the rail, however drops down to 31 in about fifteen minutes. I plan to replace the gas cap and check the fuel return system next.