Fencer

I just puchased a 2000 F-250 with 7.3 turbo diesel. Truck runs good most of the time, but then occasionally it just bogs out, no power. It does not miss on just some of the cylinders, but seems like a fuel restriction to all. I've changed the filter under the hood, no help there. Diesel shop put it on their computer - no codes, looks O.K. there. He thought maybe the batteries - so put in 2 new batteries - same problem.
I've got 4 ideas - anybody want to steer me right?
1. Plugged filters in tank - but then why would it run perfect sometimes??
2. Fuel pump on frame not pumping enough? Bad pump? Bad elec connection?
3. Slight leak in fuel line between tank and the pump on the frame, letting it suck in a little air?
4. Cam sensor bad - but they tell me they usually quit completely when they go?
P.S. I found the previous owner. He had it about 6 months and sold it because of this!! I didn't do carfax and track it back further, maybe should!
Thanks for any help!!!