Might be a flaky temp guage or sender is causing flakey readings.
The guage and sender might be reading correctly but your temp is fluctating.
Have to determine which, first.
Best to install a known good temp sensing system so that you know.
Assuming the indication you are receiving is correct, your problems may be the engine cooling temp sensor, the PCM, the radiator fan and it's associated control relay, the radiator, the water pump, or maybe even something else obstructing the coolant flow through the system. Might be ir pockets in the system, although they usually disipate over a few driving cycles.
Try to observe if there is any correlation between the cooling fan operating and the swings in the temp guage. Also, note that the AC, when turned on, should also turn on the cooling fan.
Have fun...
Steve