2004 chevy cavalier temperature gauge is all over

Check your coolant level. If you have air in the lines, the times when there's no coolant in the area of the sensor will make the temp go up rapidly, then it goes back down when coolant hits it again. You need to get the air out of the system, or you WILL have larger issues at hand, like a blown head gasket, cracked head, reduced piston ring sealing, or material transfer from the pistons to the cylinder walls. One other thing that could cause this is a faulty thermostat.

More than likely the temperature gauge has become faulty and needs replacement. Especially since the vehicle does not seem to be overheating.