Temperature gauge is constantly fluctuating. - 2000 Nissan Frontier

SOURCE: Temperature gauge fluctuates and the heater blows cold air

Sounds like the thermostat needs to be replaced. If you do the work yourself be sure the temp threshold is correct and the flow direction is correct when installed. Good luck to you

SOURCE: temperature gauge fluctuating surges

That sounds like the classic symptoms of a cracked ECTS (temperature sensor). It is located on the head near the upper radiator hose and battery. You will see two wires (likely a yellow and a black) going to a small sensor hiding in a little cubby hole. If you pull off the connector (there are two points that if you squeeze it lets the connector come off) you will probably see that the connector is corroded because the plastic body of the original ECTS cracked and let a small amount of coolant into the connector. The ECTS is only about a $10-15 part, be sure to get the updated one with a brass body. Then the hard part is you will likely need to get a new connector, one of the best places for that is a junk yard, the temp sensor for the transmission and the air temperature sensor near the air filter use the same connector. When putting this in be sure to solder the connections and cover that with heat shrink tubing. There are video's from Richpin06A on youtube that will show you how if you have any doubts. The saturnfans forum is a great place for info too,
http://www.youtube.com/watch?v=yQx84-93WkE&feature=related

SOURCE: temperature gauge is fluctuating while driving.

I'm thinking the coolnat level is low, and this might be caused by a bad head gasket - At least $800 to fix - sorry.

SOURCE: Temperature gauge fluctuates

First of all you have a bad heater core that needs to be replaced. Second you may have a bad temperature sensor. Make sure the radiator if full of water