I was headed to the store the other day when my car started overheating. It got past the third line on the temperature gauge and I pulled over. It kept climbing after I pulled over so I turned the car off. When I turned it back on the temp went back to normal. I started driving it home thinking it needed coolant and it started to heat up on me again. After I got it home I put some coolant in it and let it run for a little while and observed the gauge. It bounced in-between the normal resting point and the third line over and over. Does anyone know what could be causing this?

 

There are lots of possible causes for overheating. If you had to add coolant, there is a leak somewhere, which may be part or all of your problem. You may have a bad radiator cap. That can cause a loss of coolant. there may be an actual leak in the system, perhaps a crack in the radiator or a bad hose. Often such a leak will show evidence by white deposits where the leak is. Water pumps sometimes leak. The behavior of the gauge you describe can be caused by air in the system. There may be some air trapped in the system that will need to work its way out over time. Check the coolant level over a period of a few days and top off as needed. With normal driving, the air should eventually work its way out which should stabilize the readings of the gauge. Look for debris (leaves and such) on the front of the radiator.
Try to look for the inexpensive fixes first, like cleaning off the radiator, replacing the radiator cap, getting the air out of the system. If those don't do the trick, you may need to look to find other causes like hose leaks that will be more expensive. Hopefully you won't wind up replacing a water pump or head gasket which can be a lot more expensive. Good luck.