1998 GMC Sierra Fuel pressure question

I have been dealing with a random misfire issue for quite some time now and it has finally warmed up some outside for me to do some further work on things. Ok, I hooked a fuel pressure gauge up to the vehicle and started it and got 55 psi when idling. Warms up and then the miss shows up, usually on cylinders 6, 7, or 8. Just those three. When I shut the vehicle off within 10 minutes the fuel pressure on the gauge drops to 30 psi. Could the lack of fuel pressure generated cause my miss on just these cylinders? Or am I looking at more problems than possibly just a pump? Has new cap, rotor wires all check out fine. Any help or suggestions on this would be greatly appreciated. At about the end of my rope and don't really want to resort to the throwing expensive parts at it without really knowing what is wrong with it fashion.