2001 Eclipse GT overheating then cooling down within 20 second interval?

Last winter I noticed my car taking longer to heat up, on my way to school upon getting on the highway and upping my speed the heat would kick in. Not too mention a jump in the car's temperature rising and then going back to normal. This summer I noticed my air would get warm and I'd look down to my temperature and it would be almost to the H. If I start to watch the temperature, I'll brake and it'll rise, I'll accelerate and it'll cool down. After this happening again it did the opposite, cool down when braking... Vise Versa. Any ideas? Seems as though the car temperature will fluctuate within seconds.