K10 Running hot

[killer_mike]

Hey, have some temp issues with my truck. It has a new thermostat and a rad cap rated 13psi. The truck heats up really quick say 3 to 4 minutes running at iddle, the temp gauge returns to dead cold when the truck hasnt been started.
Read some of the old posts, im trying to find a way to determine the exact culprit, the other day i figured i could run it at night and made a trip of about 269 miles [i had to, is my only truck] the truck was mighty fine, the neddle on the gauge was a little less than 3/4 of the entire gauge reading. But when i got it out the shop, the thing was stupid hot. Im thinking about buying one of those caps with a thermometer, to determine the exact temp reading.

And then maybe replace the pump [got it from a junkyard] and take the rad to a shop; i will check the water pump to make sure is not a reverse rotation pump, i dont even remenber what vehicle or year i got it from.

Thanks in advance.