If the fuel filter has not been replaced in recent memory, it's time now. I'd put a test fuel pressure gauge on the system and leave it overnight. Then, observe the gauge when you shut the engine down and when you start it the next day. If it loses fuel pressure quickly when shut off, I'd suspect the check valve in the fuel pump or the fuel pressure regulator. You can eliminate the regulator by pinching off the return line, then any leakdown is in the fuel pump. During cold start (don't forget to unpinch the return line), you can observe the fuel pressure. It should quickly jump up to and hold stable pressure when you turn the key to RUN. Watch for signs of a faulty fuel pump.