Stalling and backfiring after driving 50 min

[REByar]

If I drive around town and keep the speed at 40 or less everything is fine no matter how long I drive. But if I want to drive to the next town, approx. 45 miles away, and drive for most of the drive over 45mph after about 50 min the car starts to sputter and backfire and the more I press the gas pedal the slower the car will go. If I can nurse it along for about 3-4 min all of a sudden it will start to run fine. If I pull it over, which I did the first two times it happened, the engine will die. It will then not start. I will pull out one of the plugs and it is covered with carbon. I will try and start it again and it will not start. The first time it happened I had to have it towed. The second time I pulled over I had to let it cool and then cleaned off the plugs. It would then start again. If it occurs and I can nurse it along and it starts to run fine again I can make it to the next town. But when I leave I can drive for about 50 min and it happens again. Any ideas what is happening? Any advice would be greatly appreciated.
Thanks
REB