Idle/cough?
 

My foreman's truck has a slight cough kinda like its loping gets low then comes right back up then clears right out.It is very short maybe 45 seconds top.It does not do this when its high idling, he runs a quart of power service through it every tank.The mods are bullydog, boost fooler,air intake baffle and muffler delete.Pack brake. It will do this from stock setting up to performance does not matter. Fuel filter is changed every oil changed (5-6k).Will try to get a sound clip loaded up the truck has broke 200k on the stock clutch and pulls 25k almost every day.Any ideas?:s:

my bet is the fuel control actuator. my 03 did that for 6 mos. before I figured it out. the part is available from cummins, costs $110 and takes 5 minutes to replace. you'll need the engine id number located on the driver's side of the valve cover.

It ended up being a bad injector from dealer actually 3.