iveco tector 75e17 2002 low revs?

my truck starts and drives but low on revs if you turn it off then restart it will drive fine for 1/2 mile then does it again its a 2002 sometimes it even runs lumpy like its on 3 cylinders instead of 4 , ive tried the normal things like filters , blown all pipes back , even the filter at the bottom of the sender , bypassed the primer and still the same anyone any ideas please