I would start with battery cables, the - to be specific.
Most common phrase heard at the stealership:
"We could not duplicate the problem" followed by a bill for the standard shop fee of $99 just to look at it
if they got under there and looked they might actually find something, but they wont evne look because basically they dont believe you have a problem if they dont see it.
Start with the basics, check voltage at the battery, check voltage at the battery when running. Check voltage at the starter solenoid, check voltage at the starter solenoid when the key is turned to "start"
First suspect for a slow start condition, especially when intermittant is the battery ground cable(s).
the Ford starter gets its - power through the drivetrain, so if the cable going from the battery to the frame and drivetrain is bad = no power for the starter.
common on these trucks.
Now there are many other things to check too, but this is a good place to start.
Also a search on this forum might get you thinking, this happens alot.
My most recent weak starter condition turned out to be another bad optima red top battery, I have owned 10 or so and had 3 of them completely fail on me to date (one cell dead = 11.4 v putput fully charged) I no longer will buy Optima's, which is a shame because the good ones last forever. $67 for a Duralast gold (consumer reports rated this battery best) and I am done.
