Truck was running fine, then started getting oil pressure dip at hard stops. All of a sudden, started to get no oil pressure. Dummy light on, lifters tapping, knock in bottom end. Shut truck down after only 2 miles of surface street driving and had it towed home. Performed engine flush twice with 2 oil and filter changes. Added Lucas stabilizer. Get good pressure when starting and idling (jumps immediately to 40 psi on stock gauge which is normal and stays during idle) Can drive 10-15 miles with perfect pressure, but after this pressure drops back to zero and dummy light comes on. Stop truck, turn engine off for 30-60 seconds, crank again and pressure jumps immediately back to 40psi. Good for another 2-5 miles. After this, when rpms go above 2000-2500, pressure goes down to zero and back up. Eventually ends up at zero again. What's going on?!! Please help!!!