Engine won't start. No fuel pressure

I have a 1995 Ford Thunderbird with a 3.8L V6 engine. For the past three months the engine will, about once per week, not start. The engine cranks normally but won't fire. Sometimes it will start in a few minutes other times it won't start until the next morning. When it won't start I don't hear the fuel system pressurize. The fuel pump, Integrated Relay Control and Idle Air control have been replaced with new parts, but the problem persists. What else could be the issue? Thank you!