Ok so my buddies 2000 2500 has a fuel pressure gauge. He just picked it up 3 weeks ago. It's been fine until 2 days ago, his truck started stalling. As his fuel pressure went from 15 to 2psi under load. Unless he is only idling the fp sticks around 5psi. Sometimes lower. He's replaced the lift pump, fuel filter and filled his tank. And it still does it. Any possibilities of what the problem could be