I have a 93 Full Size Bronco with the 5.8L V8. Several months ago after driving for about an hour (mostly on the highway) the car began to lose power and eventually would only run at a low idle for the last few minutes, then stalled (luckily right in front of a friend's house). Being in a rush, I left it there until the next morning. It started up fine and ran well at low and high RPM's out of gear for several minutes, but soon after putting it into gear it began to lose power again and eventually stalled again. I left it for a week, at which point it started fine and ran well for the next several months.
A week or two ago, the same thing happened. It slowly lost power until it would only idle after driving for several hours on the interstate. It went down to a real low idle and was running very rough. Under the hood I could see the engine shaking hard. It eventually stalled. 15 minutes later it started, ran well for a few minutes, then lost power and eventually stalled again when I put it into gear. Same thing again 10 minutes later. An hour later it started fine and has been running just great for a week or two now.
A dealership looked at it, claims to have run whatever tests they could against both the fuel and ignition systems, and found nothing wrong.
Now I've got a great car that I'm afraid to take on a road trip since I have no idea when it will stall again or how long it will take until it feels like starting again.
Ideas?

 

I have seen something like this happen on a diesel once. The fuel oil filter had a very small hole in it and it would eventually cause the engine to shutdown.

 

I'd change your fuel pump and filter and go from there.