2002 Nissan Maxima Coolant running hot

I noticed that smoke was coming from under my hood when I stop at a traffic light, so I stopped and checked to see what was causing it to smoke. I noticed that the coolant was leaking on the radiator. I put some stop leak in it, I also check to see if the car was running hot by looking at the temporature gauge, and it is not. The hot air runs just great. I drained the coolant and flushed the radiator, then I refilled the radiator, and let it idle for about 15 minutes with the heat on high. It ran okay for a while, but I noticed that coolant is coming out of the radiotor cap. The car is not running hot.