Car over heating

Sounds like you need a new thermostat. While you are at it flush the engine and put new anti-freeze in.

I'm not sure that you have a problem at all. While it does seem a bit extreme, I think what you are seeing is the engine heating up, the thermostat opening, the temperature dropping, and the thermostat closing again. That's a pretty normal cycle. Having said that, thermostats don't last forever and they aren't expensive so change it out just to be sure. And as has been mentioned if your coolant is 5 years old or older its time to flush the system and put in new coolant. Change the plastic elbows that run to the heater while you're at it. They don't last forever either and will leave you stranded when they go.