When your engines temperature gauge constantly fluctuates and doesn't stay consistent then it's most times caused by a faulty thermostat on your vehicles engine.
The thermostat for your vehicle is located on the engine and inside a connector that connects your top radiator hose to your engine and is usually located on the intake manifold but could be located elsewhere depending on the make/model of vehicle and engine design.
When the coolant/engine thermometer sticks either in the open or closed position the temperature gauge in your vehicle will go up and down and not stay in one position.
Having a clogged radiator or even a bad water pump can cause temperature gauges to fluctuate in your vehicle as well.
Either way it shouldn't be happening and it's an indicator of an engine cooling problem and needs fixed before you have an overheated and damaged engine.
You can take the engine thermostat out and boil it in a pan of water and if it opens then it's okay but if it doesn't then it's bad.
Just be sure to not use one of your good cooking pans because you can't cook in the pan anymore once you put the coolant thermostat in it.