I'm a bit late on the reply, but it's more than lightly a wiring harness problem. My 6.0 had a very similar problem, would not start and after 10 or so failed starts would finally throw a cam censor error code or cam / crank out of sync error code. It would only ever do this randomly so was a real pain to diagnose. After a period of time I found that I could reach down to the wiring harness that runs under the coolant reservoir to the FICM give it a tug, and all would be fine for the next 2~5k miles or so.
The FICM wiring harness is not all that expensive, but replacing it is a nightmare. So I decided to pull it apart in-place and see if I could track down the issue.
There is a ton of information on this issue on the 6.0 PSD trucks, and a lot of it is the same for the van. My problem / chafing location was described in a Ford TSB. (First image of page 5 in the below link)
I couldn't find a neat way to stop the harness rubbing on the manifold bolt. I first thought about making a semicircle bracket that bolts to the manifold and acts as a cradle for the harness, but ruled that out as I could only make one from metal and I was worried about heat buildup. So in the end I zip tied it (with quality ties that will stand up to heat), to the engine lift bracket you see in the picture. Not shown in that picture is the second fuel pump location on the E350, I also put a rubber sleeve about one of the metal fuel pipes and zip tied it to that. Both of those locations raised it 1/4 inch off the offending manifold bold.
This URL is also a must read to diagnose the issue.
http://www.powerstroke.org/forum/6-0l-p ... -here.html
I had stripped all the harness sleeving material back from the FICM to this point to find the issue, it's about as far back as you can go without pulling the harness from the vehicle. But if I was doing it again, I'd go right to that spot and start looking at it carefully. You will need to remove a bunch of stuff to get their though.
Move Coolant reservoir out of the way.
Put firewall harness out of way (same procedure for upper/small filter change).
Move power steering reservoir
Pull intake tube (the one that runs from air box to intake).
Pull turbo piping (right side of radiator, and upper one).
Probably a few other small items.
Then you can unplug harness from FICM, then ECU, and few other spots, you should be able to untangle it from all the other items without removing them.
All that will give you JUST enough room to work on that spot.