Low fuel pressure Duramax

I would check the O-Ring, check the WIF sensor for cracks, or the nylon bleeder screw could be worn. If those are good, do you know if the fuel filter was ever overtightened? If so, it may have started a small crack in the fuel filter housing. I have replaced a few on buddies Duramax's that either tightened them to much (Hand tighten only)and cracked them, or took it somewhere where someone else overtightened it. They can be had for around $125 online.
BobsYourUncle - My BIL truck was doing exactly what you described. I replaced the fuel filter housing and he has not had an issue since.