sustained high speed causes near stoppage

When traveling at highway speeds for two or three hundred miles, the van will all of a sudden act strange and begin to slow to around 30 mph and will back fire occasionally. After stopping for a while, it will start fine and run again. One time, it got so bad that it was almost to a stop before we could get off of the freeway. Our mechanic is buffaloed! Help!

Try replacing the fuel filter, and make sure the gas tank has no debris in it that's blocking the fuel pick up tube.