1997 Honda Civic My car is overheating

Hi,
I have a 1997 Honda Civic, 2 door, DX, automatic (160,000 miles). For the past few months its been overheating and the temperature gauge starts fluctuating, goes over the half way mark towards the redline, but then when I start driving, it goes back down to the middle, then when I stop at a red light for a minute the temp goes back up. The only thing I do in situations like this is pull over and refill the radiator. I have noticed that when the car is overheating the coolant seems to get low in the radiator and keeps filling the overflow tank. Why does the radiator show low coolant? And why is it dumping it back in the overfill tank when I’m driving. This is what i`ve done on the car:
Regular oil changes
Transmission flushed 3-4 times for the life of the car
Timing belt & water pump recently replaced
Radiator Cap recently replaced
Thermostat Recently replaced
Radiator Hoses Recently replaced
Radiator has no leaks ( I don’t see a puddle on the ground)
There is no white smoke coming out of my car’s exhaust
Why is my car still overheating? Any help would be appreciated.
Thanks!