N14 Redtop Cummins Skip

UPDATE
Ok. The fuel pump has been checked. It is fine.
We are honing in on it but I'm hoping with this new information, someone out there might have something to add.?
We took the truck to the shop that replaced injectors # 5 and 6. 5 and 6 are on the rear head (#3) on an N14.
They inspected the O Rings on their installation to make sure they were ok. They are fine.
BUT #5 injector had some carbon build up on it and this injector is hardly a month old.
They cylinder was dry. The mechanic tested the compression in 5 and 6 and the compression in cylinder 5 was about 70 lb's lower than the compression in #6.
He also found a decent amount of oil around that third head...on the rear side Since we were under a load at the time, we didn't want to pull the head to check and see if we might have a crack in the head.
So this is where we are.
To get the truck to run right after a sputter, the driver has to take the RPMs up to 18 or 1900. After doing this, the power comes back and it runs smooth again.
We have a prime issue...The truck does not want to crank after sitting for a night or so. It is hard to crank.
#5 cylinder has a lack of compression.
#5 injector has some carbon build up after only being in the truck for a month or so.
We have an oil leak issue on the 3rd (rear head).
Our next thought is to pull the head and inspect it for cracks or maybe a blown head gasket.
The entire fuel system has been checked and the entire electrical system has been checked.
Everything is ok. If the problem was constant, we would consider a mechanical issue but it comes and goes.
Any ideas, please throw them out there. I can't thank all of you enough for chiming in.

 

Thanks for the replies 062, Bigguns, and Tony. I didn't mention this before because I didn't want to go too far into the year long back story and bore people to tears. #5 has been replaced twice in the last 6 months. #5 seems to be the issue here...no doubt. Why is that injector in #5 (the newest one) already covered with carbon after only a month? The oil leak, though is new...the last month or so. We went over the entire list today of all of the suggestions that people have given us on here and they have all been applied excet the pass through connectors. 062, just wondering why the cooling system leak down? Tony, I agree. I think we need to replace the pass through connectors first. Many people have recommended that and we have not gotten to that yet. I'll see if Cummins still carries the flush but I don't think that will cure this. If the head gasket is blown and or if the head is cracked, would that cause a lack of compression on a test? We will get to these new recommendations asap. Thanks for all the help and suggestions.

 

Well, we got the news today. The truck is at Associated Fuel in Atlanta. The entire engine must be overhauled. Bad cylinder walls, bad cam, crank must be polished...they found evidence of water in the pan. The rings were bad. So....there we are. What was the actual cause of the skip? Not entirely sure but there were numerous serious issues. I guess I'm glad to know. Everything that I learn, I have to learn the hard way. I will NEVER purchase a "low mile" drop in used engine again. We got a year and a half out of it but after paying for everything, that POS was around $12K, including installation and down time and nothing but a constant worry for the year and a half. We're gonna spend the $21k and hopefully will have a great motor with years of life ahead of it. I guess this skip would have lead to a piston and rod coming out of the side of the block. Thanks to all that replied and tried to help. I will update as soon as the truck is back on the road. It should be about a week and a half.