Air on the suction side of the lift pump would be where it just doesn't run. Air between the lift pump and IP is where it gets tricky. Since air molecules are smaller than diesel molecules. You can't see a small leak that lets air in. Most people have issues with this kind of leak if they park nose up. The engine will start, run for 5-10 seconds, surge faster and then die. 30-40 seconds of cranking will be required to get things primed again and all will be well until the next random time it happens again on cold start up.
Have you pulled the return line fitting from the IP? Standing at the bumper there is a black hose with a red squeeze clamp on the top of the IP running right at you. Remove the hose and use a 9/16" socket to pull the fitting. Look inside the fitting. There should be a glass ball with a spring behind it. You should be able to see through it. If there is any little black pieces in there, your IP is about to die. If the glass ball is gone, someone else already found something in there and knocked it out so the truck would continue to run for a while.
A general knowledge tidbit here. If the truck will not turn off with the key some day. Just use a pair of pliers to squeeze that black return line. The pressure build up will kill the engine without any harm to the engine.
If your glass ball is there and clean. Then, check out the return line all the way to the fuel tank. Make sure it isn't kinked or something somewhere. You can remove the fuel cap and glow very low pressure air through the line, 5-10 psi, with someone listening near the tank, not face in the fill cap.
One of my trucks had a restriction in that line. It blew the rear injector return caps off. I thought it was a return cap clamp problem. The engine would lug a little and when it started running better, I would know when I was blowing diesel all over the place and the cap was gone. I finally figured out it was the return line pinched near the fuel tank.
