Twice now my F-150 has just died on me as I was driving down the freeway.

So twice now my f150 has just died on me as I was driving down the freeway. Both trips were about the same duration but all guages were normal. Attempting to start the car immediately after the stalls did not work, however, both times after the car sat for approximately 1/2 hour the truck fired right up and drove fine. On the second time it stalled attempts to start the car immediately after the stall resulted in very weak/sputtery firing and then stall, but after 1/2 hour of resting it fired right up.
My mechanic found no codes coming up on the computer and idled it in the shop for the better part of the day and could not replicate. He suggests it might be the feul pump, but this is an expensive guess.
Any help from anyone out there would be greatly appreciated.