1. Check valve in fuel pump not working causing fuel to leak back into the tank. Try replace the fuel pump. From what I understand it's ok for the pressure to drop, but as long as there is still fuel present in the line it's ok as pressure is restored once you crank.
2. Maybe there could be a restriction in the fuel filter causing problems, try replace the fuel filter.
3. The cold start injector could be leaking fuel into the manifold slowly. To check this take it out, turn the car on for a while then turn it off, you can then watch for signs of fuel.
4. Fuel Pressure Regulator.
5. Injectors leaking fuel over night, clean injectors.
6. It could be a possibility that valve stem seals are leaky, causing oil to pool up into the cylinder and when started it may have a hard time starting.
7. Maybe the fuel pump ECU, I remember people having problems with starting the car while it's warm, what about cold?
