Lack of fuel?
 

Ok so I'm new to diesels. 99 cummins when driving maintaining speed chugs like its sucking air but give it some throttle and it goes...also some hard starts and with fuel cap on builds pressure or vacuum in tank....new ip new stock lp all new sensors bumper to bumper...also not always black smoke on start up and no smoke when driving:( please help I have spent lots of cash and time to still be at square one

I've noticed sometimes when i have my fuel cap on to tight i'll hear a hissing when i remove it. Maybe yours is on to tight sometimes. Just a thought.
Also on those years of cummins people that i have talked to have upgraded their fuel lines going from the fuel tank to the lift pump then to the inj. pump due to the stock lines not being big enough causing a lack of fuel to the inj. pump burning up inj. pumps.
I think it was going to 3/4 or a 1/2 line.

First of all ditch that stock lift pump! Get yourself a fuel psi gauge and save from killing that new ip.