But to me this sounds like some blockage or a failed electric fan on the radiator. You'll want a service manual to see how to test the fan but the blockage or possible value would be more likely a service shop repair.
At this age it may be time for a flush and have them test that thermostat.
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 am having the same problem. It actually has got worse when I am 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 im 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.