I had a similar problem on my '00 Civic. On two or three occasions it was overheating so I pulled over, noticed the coolant was low, and topped it off. I changed the thermostat myself, and it seemed OK. I was able to drive it 250+ miles in one day with 4 of us in the car with no problems.
Then one day after I'd driven around for a good 120 miles, I stopped to do some window shopping about 30 miles from home. When I got back on the road, the car started getting hot, so I got off at the next exit. I happened to be there when a car show was going on, so I parked the car and walked around taking pictures for almost an hour. It was a hot day though, with temperatures in the 90's, so letting the car rest didn't help. I topped off the coolant with the last of what I had, and got back on the road, but it overheated again. This time I had to drive it like that a little further to find some shade to park in. When I popped the hood it was steaming like crazy, the coolant reservoir had started melting and the coolant vapor had made a hole in it. Eventually I got some more coolant in it and drove the rest of the way home.
Anyway, I got that reservoir replaced with a cheap one from ebay and took it to a mechanic my cousin suggested. It turned out to be the radiator fan that wasn't working, so even though everything was flowing fine the fan wouldn't turn on to dissipate the heat. I had suspected that could be a cause, but would have had to cross some wires to find out. While he was at it he changed the water pump and timing belt and a few other belts and it's been fine for the 30k miles since.
One more thing to note: I believe on these cars the temperature "gauge" isn't really a gauge, it's actually only holds two positions. If the car is fine the needle is level, otherwise it shoots up to the max and the light turns on. That makes it harder to diagnose since you can't tell if the car is "getting" hot or how hot it's gotten.