It sounds to me like whoever owned the truck b4 you has replaced the motor with one that was for a truck with no intermittent function option(they're cheaper). If you go to the local part store, they will be able to tell you how to tell the difference between the two. Also, they should be able to test or tell you how to test the motor assembly. A new (remanufactured one) is relatively cheap (around $25) chevy wiper motors are prone to failure, they are one of the highest selling ones at the part store where I work.
It sounds to me like yours is either rusty inside or worn out, which is why they will work for a while then the motor overheats and cuts out until it cools down.
If you don't already have one, get a good shop manual. It will explain it more in depth. A manual is the best possible tool you can buy when it comes to car (or truck ) repair.
Hopefully this sheds some light on to your particular problem.
