Your issues sound like how my '83 6.9 ran when I bought it, that truck was a farm truck so it didn't have many miles but it had a LOT of idling time. Since it only had 62k on the clock I put a set of new stanadyne injections in it and it woke it up, if it had been closer to 100k I probably would have done the injection pump as well. Personally I'd try a new set of quality injectors and see how much it improves and then decide whether your injection pump needs a rebuild or not. Also check your line from the frame to the lift pump, my granddad's '88 7.3 was a dog after he had the lift pump replaced and come to find out they used regular gas hose between the frame and lift pump so when you drove the truck on the road it would suck the line flat and starve for fuel, it would never quit running but it was severely lacking in the giddy up department!