Alexio Auditore

I've owned my Dodge for over 4 years now, and have upgraded the lift pump, which is the raptor 100 gph on the side of the block, edge ez, 4 inch exhaust, and I believe it has at least 60hp injectors that were installed by the previous owner. I am at 400 rwhp now, tested at the hunting for horse power up in cda. My question that I am asking, why is my fuel pressure dropping down 5psi at WOT? I have spent many hours trying to find information on this, and its hard to get a definite answer. the Diesel mechanic I take my truck to says its normal, but I know its not, especially since my truck will start to stutter if I let off the pedal after stomping into it for to long( by no means do I drive the truck aggressive, I drive it like an old man, like my old man lol) I want to upgrade my fuel lines to half inch from the tank to the lift pump, including fittings. but when I look online, I'm only finding the big line kit from the filter to the injection pump. Any advice would be much appreciated. I'm not looking to start a proper fuel psi forum, as I know its been beat to death. 15 psi at idle, 5 psi at WOT- doesn't seem normal. I'm on my second raptor 100gph -----this is my first post, let me know if I'm doing anything wrong.