Temperature guage goes up and down

What is your question?

My Volkswagen Passat V6 temperature guage, or coolant guage goes from normal(the needle is in the middle), then as I'm driving the guage goes down in temperature (toward the cooler part of the guage). There is no sign of a leak on the ground where I park it, and it's not overheating and no check engine light is on. I'm confused about what is wrong with the temperature guage. Can someone offer me some advice as to what this problem might be please??

What makes this problem better or worse?

When I add coolant

How long has your 2001 Volkswagen Passat had this problem?

Month, but I don't drive much