radiator, crap block air through radiator
hoses
water pump
buying wrong thermostats
incorrect coolant, amount or mixture
fan
fan thermostat
engine running hot -- bad timing, fuel, or mixture
drag on engine, AC or other accessory
drag on entire car while driving;  brakes, driveline

Lots of things, including the water pump, clogged lines or radiator, or
a bad temperature sensor leading you to think it's not overheating when
it is.  What is it doing when it overheads and is the fluid level low
when that happens?  if you look down the radiator do you see lots of crusty
white stuff?
--scott

Lets add cracked head or block, or blown head gaskets to the already
helpful lists.

Hi. You could have an air bubble in the coolant. Some cars have a plug
or screw to bleed air high up in the cooling system. Check your manual
or call a dealer service department and ask if your engine has one. If
it doesn't you could disconnect one of the heater hoses from the
heater core (if it's higher than the radiator cap). Hold the
disconnected hose at the same level as the radiator cap and fill the
radiator until coolant comes out of the heater hose. If you don't have
air in the system you could have a bad radiator. Good luck.