Hi Guys I have a 2008 focus 1.6 tdci econetic

I have only had the car a month but I notice the temp gauge goes to the middle which I assume is 90 c and then drops to 75 c, and then fluctuates between 75c and 90c

I tried the test mode and the engine temp read 120c and drops and increases when the gauge does

Should this stay constant or does it appear to be a problem ?

Thanks