Car overheating and now dieing.

I own a 97' chevy Cavalier and it has ben over heating. It has around 115000 miles on it. I flushed the radiator and noticed signs of stop leak in it. I can add water to the car and let it sit and there will be no signs of a leak. Once I run it for 20-30 minutes though it will over heat and be completely out of water. The belt seems ok but could be replaced. There is no signs of water in the oil at all. Simply my problem was when I got it the temp gauge sat at 3/4. After several months it crept towards hot. Then it got to hot but ran ok. Then once in a while it would go to hot and the check guage light would come on and then immediatly the temp guage would drop back down to 3/4. There was no physical signs of overheating though. I checked everything and though it could be a guage or sensor. There is also no thermostat in the car and had none when I bought it.