Why my Temperature gauge keeps going up and down?

I have a 05 Honda Civic EX, my temperature gauge has been going up and down. I thought that maybe it was the thermostat and had it replaced but I'm having the same problem. It actually has got worse when I'm driving the gauge goes up but when I let off of the gas it goes back down but once I start pressing the gas it will go up again. When i'm at a complete stop it goes up slowly too, i notice when i have the heater on once it starts blowing cold air the temperature raises. I checked the coolant and its fine so that's not it.

Read more: Why my Temperature gauge keeps going up and down?