Need some suggestions:

I have a 2002 F150 with the 4.6 engine.  Last June my wife came out of a
store and the truck would turn over fast and normal but would not start.
  It wouldn't fire or anything.

After letting it sit for about 45 minutes it started right and ran just
fine.  I replaced the fuel pump relay with a new one thinking that it
might have just been sticking occasionally.

We have driven it over 1500 miles since that time without a single
problem until today when the same thing occurred.  We let it sit for 30
to 40 minutes and it started right up and will probably run  fine for
another 4 months without incident.

I'm at a loss but was wondering if anyone else ever had a similar
problem?  Or what I could look for.

Thanks in advance