Temperature gauge on 2001 Blazer going all the way over than back to zero. Sometimes works perfect

 

while driving?
any particular situation?
reproducable?

Please refrain from using a bold typeface in the writing of the thread. better readability without.

 

Typical for a bad ground. The sensor (whatever it is) is a variable resistor across the signal line to ground, when the ground opens, the sensor line has full voltage. Could be corrosion between the sensor body and whatever it screws into, or a bad sensor. If the ground from the engine to electrical systme was bad, that might harm the ignition, so that isnt my first guess.