Please check for codes, while not all problems on OBDI can be picked up, it is the first place to start. Beyond that, if no codes, then check fuel pressure at the rail using the Schrader valve. Rent a tester from a part house if need be. Check pressure at idle and again with the vacuum line removed from the fuel pressure regulator. Reinstall the vacuum line and then shut her down. Continue to monitor the pressure over time and see how much it bleeds down if at all. This info will give us a good point to start with to help you solve this issue. This may not show any good info, but at least many things will be ruled out.