2002 Honda Civic Overheating

I have a 2002 Honda Civic Si. It has 138,000kms on it. Last weekend, while camping, I didn't see the temperature guage raising up, so it overheated and blew the cap off of the overflow resevoir. I topped up the radiator, and the resevoir, and it slowly cooled back down, and worked fine for a few days, I went and got a radiator flush, and it worked fine, but now it's doing the same things again. Most of the time on the highway, it seems to stay cool, but not all of the time. Any ideas on what the problem might be? Also, the resevoir stayed full, until the last time it overheated, and not it's really low. Any help or ideas would be greatly appreciated. Thx!