Fuel Pressure question

Anyone know what would cause the fuel pressure to be good while the engine is cold but it drops a lot when its warm, I having a long start when the engine is warm and fuel pressure is real low when its hot, it builds up when starting and starts up with abit of cranking, and when its started it seems to hold static pressure.
Any ideas?

year, engine???

1998 Ram 1500 4WD 5.9L