Temp gauge works then quits

I have a 96 cavalier, it was starting to overheat and the fan wasn't kicking on, so I replaced the fan motor. Now the guage works properly when I first start driving and heats up to normal. Half way and then drops to zero and stays that way,usually, sometimes it will start working a lil sometimes not. It sometimes acts like the needles on the old type floating gas guages.. I check to make sure it has the right amount of fluid and isn't overheating and is allways ok. I'm just afraid that one of these times when it registers cold that it is really overheating and gonna burn up my motor. Temp. guage always worked perfect before I had to replace cooling fan. Does anyone have any ideas?

First thing I'd do is inspect all wiring connected to or in the general area of the replaced cooling fan motor to make sure something wasn't left unconnected or accidentally unplugged ! If all is okay there , I'd replace the engine temperature sensor . It's relatively inexpensive and likely the original one so replacing it is a good idea anyway ! Also make sure your coolant level is where it's supposed to be as a low level can cause the temperature gauge to do strange things . Last thing I'd check is the thermostat as it can cause the temperature to act erratically ! Good luck and keep us updated !

I'll try all of your suggestions, starting with the sensor. I'll be in touch.