[alloro]

1. A bad sender.
2. A bad gauge.
3. A restricted radiator.

First you will need to verify that it is really overheating by using a thermometer. You can check the radiator by feeling it from top to bottom and see if it's evenly hot. If you find a cool section then it's likely the tubes in that area are plugged. A back-flush might solve it, but more than likely a new radiator will be in order.