Rail pressure not steady
 

I have an 05 which is modded a bit, but my problem is: rail pressure will move up and down a 2k-3k psi pretty much any time you need to be in the throttle. Relief valve? I also read about people having issues with the FCA, what do i need to check, look into if it would be the FCA? truck has dual cp3s and flux 4s. any ideas or thoughts greatly appreciated.