1986 Chevy Blazer Temperature Guage

The temperature guage in my 1986 Chevy Blazer reads past the red zone all the time, even when the truck has been cooled off. It started one day while driving, the guage was reading normal temperature when it instantly clicked over past the red zone and stayed there. It stayed that way for a while, regardless of the truck's actual temperature. Months later however, while driving, it suddenly clicked back to reading the appropriate temperature. This lasted for just a few minutes while driving, then went back to reading past the red zone again. My question: Is the gauge faulty or is the temperature sending unit faulty. If it is the temperature sending unit, where exactly is it located on K5 blazers with the 5.0L engine?
-Josh