So my dad has a 2008 F550 6.4. When I go from a stop or low speeds to high speeds, at around 40mph the truck starts to hesitate. I’ll stay on 40-45 for about 10 seconds with the pedal firm to the ground. Even the turbo psi only goes to about 20psi. Then after those 10 seconds, it finally kicks in and speeds up. I’m not sure what it is. I had a duramax with the same problem and I just changed the fuel filter and it worked again like normal. Trying to see if anyone can help.