P0148 and P0251.. Help please.
 

So I have a 2007 5.9 with a six speed. The pickup is pretty well all stock. I'm getting P0148 and P0251, if I try clearing them, they come back within a few miles of driving. Sometimes when I try to take off, more so when the engine is cold, Ill get a dead pedal, and it blows some black smoke. I've had my scan tool plugged in when it does it, and I drop from about 7k in rail pressure to 3-4k, then when the pedal comes back it jumps back to 7 and up. Checked my fuel supply pressure, I had about 13 psi at ide, and about the same at WOT. I have tried changing fuel filters and the fca with no luck. Has anyone had this issue or anything similar? Looking for any suggestions or ideas, any help is much appreciated.