Does it start up again straight away?
A friend of mine had a similar problem although it was more to do with distance travelled rather than a specific speed.
After about 10/12 miles his car would splutter and die.
He would wait 5 mins or so and the thing would fire right up again,10/12 miles later, same thing.
He traced it to a blocked gas tank breather,if you released the gas cap you could hear the tank sucking in the air.
Once the tank had reached a certain point the vacuum would over power the the fuel pump suction and it would stall out.
I'm wondering if this speed in your truck is related to the fuel demand of the engine?
Might be worth checking.....
