What would cause my land rover to overheat at times?

My 2002 land rover discovery overheats at times, which I think is strange. So, I'm just wondering why and what would cause this? Drove the car yesterday, went to park it, when all of a sudden the temp raised fast, light came on dashboard, and anti freeze was just pouring out fast from the over flow tank. Turned the car off immediately, waited, filled it up with water, only thing I had, and drove it back home and everything was fine. I don't get it????|||The first thing I would change, is the thermostat. It sounds like that might be sticking at times, and may be the culprit.