2000 Explorer won't crank or start sometimes after driving it for awhile and parking

First happened when the vehicle was fairly new but not as often.  Now it happens fairly
often.

When the Explorer is driven around enough to bring the engine temperature up to full
normal temp, then parked for 10 or 15 minutes it won't crank after that.  Waiting 20-30
minutes or so it will usually crank right up and start with no problem.  The hotter the
weather is, the more often it happens and the longer I have to wait before it will start.

It happened once when I was getting the tires swapped, but started working again
while the guys at the shop were still trying to diagnose.

What would be the possible things to check?  I should have made Ford fix this early
on, but whenever I took it in to them it always started up no problem, so it is definitely
intermittent.