Well, it's either running out of fuel or has an ignition problem. An ignition problem can occur after the components have time to warm up.
What does the truck do when you give it more gas while driving it when the problem occurs? Does it bog? If it does, I'd shut off the ignition key and pull over. Work the throttle and see if the carburetor is empty or not by looking to see if it's squirting fuel still. If not, you've got rust/debris in the tank or possibly a bad fuel pump.
Of course it could be ignition. Easiest way to figure that out is to trade a distributor and coil out of another good running truck. If that is not available, possibly the condensor or ignition coil is bad. A condensor is very cheap and easy to replace. I've seen bad "New" condensors.
Good luck.
