Help needed
 

Having trouble with o2 lb7 truck have had it a few years now And its really giving me fits i had too replace the fuel lines from the tank up too the engine and since then it will kick on the check engine light and wont go past 2000 rpm this dosent happen all the time but after the lines were replaced it started it and i took too a freind who put a tech 2 scanner on it and had a low rail pressure and over/under boost code he cleared it and was fine for a couple days then i turn a corner and it does it turn off truck for a few minutes and back too normal now its done it several times when i have had too get on the brakes kinda hard im stumped the mechanic says injection pump i think if pump was bad it wouldnt run at all any help is appreciated