Overheating

Make sure the cooling fan comes on when the engine gets hot, before it overheats, usually around 210 degrees F or so, that the coolant is full and if all is okay, try a new thermostat.
If the exhaust tailpipe blows out any white smoke though, you may have a blown head gasket.

Check that the thermostat is working and that there are no defined hot and cool spots in the radiator. This will indicate a part blocked radiator which will affect circulation and cause over heating as well.