2002 Santa Fe 2.7 4WD Fuel pressure drop

Okay so Ive replaced the entire fuel system in this car, Once warmed up if you are driving in stop and go traffic and you give it a fair amount of throttle the fuel pressure will start droping getting progressivly worse as you continue to drive in traffic. The fuel pump is brand new, lines are all pressure tested, the fuel hose comes straight from the pump to the injectors and regulator, this is a return system. during my testing i have a booster pack hooked directly up to the pump so i know its getting power. this is the 3rd Fuel pump ive tried (gotta stop going to wreckers, always second guessing myself) Fuel trims are perfect right up till it actually hits 0-20 psi. Maybe Hyundai Fuel trims are garbage for some reason? its acting like the regulator is just dumping all the pressure pinching the return line it does the same thing though. O2 sensors reading Lean during the 0 fuel pressure issue. Link to a video of the issue: https://drive.google.com/open?id=14ckxFKv_c37Svh2jXplMbp1IA6e8QSbD

-New fuel pump and filter (Exact same problem)
-swaped out the fpr with one from the wreckers (Exact same problem)
-Swapped out injectors with ones from the wreckers the first time since they were leaking internally, fixed leak, (Exact same problem)
-New crank sensor
-New spark plugs and wires (previous owner)
-Replaced the timing belt