try burping the coolant system. park the truck on a hill, pop off the radiator cap (do this when the truck is cold, otherwise you'll likely burn yourself), rev the motor to about 2k rpm, and hold it like that for a little bit. (maybe a minute or two) this should help purge all the air from your cooling system.
release the throttle slowly, as doing so quickly can cause antifreeze to shoot out of your radiator. (and that's never a good thing!). top it off with coolant, put the cap back on, and see if it happens again.
if so- you have air getting into your system somehow. check all the coolant lines for pinholes (yes, a pinhole will do it), check your radiator cap like someone said, make sure your resivour is full, and if all else fails, do a leakdown test!
-shaeff