Temp gauge reading super low?

So my jeep has been running just fine. Temp gauge was working just fine. Then about two days ago it started doing weird things. After the jeeps been running and driving the temp will stay about 120 on the guage? Once in awhile when I take off from a stop light quick the guage will shoot up and read normal 190. But will drop again shortly. Whats the problem. My heat seams to still be lowing just fine. How wise I mean. I dident notice a change in it not blowing hot air anymore. Could it be my thermostat? Or is the gauge is just sticking or what? Its a 2000 with a 4.0 and AW4 tranny. Everything under the hood is stock.

Check the connector for the gauge sending unit, make sure it's tight and the wires aren't frayed. If the wiring looks good, replace the sending unit itself... it's cheap. It's at the back of the head on the driver's side and has a one-wire connector going to it. Mine did the exact same thing, turned out to be a bad sending unit.