2004 Honda Civic when driving temperature gauge moves up and down slowly when driving?

More info needed. Are you saying every time you drive your car the temperature gauge starts at low, goes to medium where it's supposed to be, and then goes back down to the very lowest reading and stays there? Or some different behavior?

That's caused by an air pocket in the cooling system. Find the leak.
I'd also replace the thermostat and antifreeze if you haven't done that in the last 3 years.

Check the radiator fan relay. You likely need a new one. The fan should turn on and off as needed. If it won't turn on while idling, but turns on when you use the AC, the fan relay is bad and the fan is good.

As long as its NOT overheating a little fluctuation is fairly normal....Its either the Thermostat opening and closing, OR the electric fan kicking on and off...More than likely there are two fans....When you turn on the A/C, OR the Defroster the A/C Compressor should cycle on & off, and one of the fans should kick on for a few minutes, then cycle on & off as well....As long as its not getting hot, or overheating this is normal....

Hi yes that is correct as the coolant is released from the main part of the radiator into the head and back when it is cooled.

Load can cause that to happen, and as long as it does not overheat it does not matter.

It's supposed to start cold, then come up to medium as you drive. It may rise further from medium to hot if you get stuck in traffic or on a hot day. This is all normal unless it goes all the way up to the red overheating zone, at which point you should move out of the way on the side of the road, switch the engine off and let it cool down before trying to drive to a mechanic for a diagnosis and repair.

If, however, you see the temperature gauge moving up and down in the medium zone as you drive around, that may indicate a a battery charging problem: perhaps too much voltage from the alternator as you speed up and too little voltage from the alternator as you slow down, causing the needle to move even if the engine temperature hasn't changed much. This will need and electrical diagnostic test.