I also had a 2005 F250 with the 6.0. Just before I got rid of it, it started being hard to start. Turned out to be the FICM. Not cheap. Repair cost at a Ford dealer 3 years ago was $1100. The FICM voltage can be read on the dealer's computer, so they don't have to guess. Most dealers do not have a decent diesel wrench, I had to drive 40 miles to get to a good one, and there are probably 6-7 Ford dealers closer than that. There are aftermarket FICM's available a little cheaper and of better quality, if you can do the repair yourself.