isx cummins is stuttering

I'm new to this thing so y'all bear with me. i just bought a 2011 Peterbilt 389 with an ISX CM2250. the engine was rebuilt 2 years ago and fully deleted at the time. when i go the truck home and got it on the road i found the fuel pump was leaking oil from it's gastket. Last weekend i removed all the lines moved the pump back about 5 inches from the front cover removed and replaced the gasket. got everything bolted back up on Saturday and was ready to work on Monday. It ran great on the first load I hauled with it. apprx. 160 miles or so. When I left that site, i got some minor hesitation and sputtering from the engine. No codes or check engine lights. Day 2 started ok for the first couple of hours and then the same problem.It seems to be doing this loaded or unloaded. Usually when starting a pull, (does exellent durring the pull) and when coming out of one. I've since replaced all the older fuel lines with new ones. Still the same. Although, its seems to be a bit better today, it's still not like it should be. Would a leaking injector cause this reaction, or is it more of an electrical issue? It feels like it's sucking air in the fuel system, but there are no leaks present. I've also presureized the system and nothing. Had it going down the road last night with a tech in the passenger seat reading the fuel pressure and other various thing with his diagnostic cmputer and everything read normal. Even he tought it was crazy. Kind of at a loss here and don't know what else to do. Anyone got any ideas? Thanks