Fuel Pressure Fluctuation

[B model]

I have a 06 cm870 500/1850 917k miles on it and it has recently developed a kind of miss when idling. I just checked and got a few bubbles out of the back 3, so it has a injector bad. Anyway my real question is, how stable should my fuel pressure be? It fluctuates so much at idle you cant even read the gauge, and at about 875 rpm it will kind stabilize enough to read about 250 psi. As for cranking pressure it just goes up instantly and starts when it gets close to 100. Is it supposed to show a even pressure or is this normal. I guess it should also be noted this truck has never had valve cover off or actuators off or any of the normal stuff until now. My mechanic assured me that you never needed to set overhead unless the cam or injector went bad. But after reading on here I decided to get in there and have a look.