2000 Ford Focus ZTS keeps over heating you have changed the thermostat hoses radiator cap and it is not leaking anywhere why does it keep overheating?

IT could have an exaust leak somewhere or if it has an electric cooling fan the fan could be bad, short in it wiring or blown fuse Is your aquastat working? Is the radiator clogged up with sediment? If it has overheated before, is the head warped? Your question is a lot of questions. Take the radiator cap off with the radiator full and gun the engine while it's still cool. If the water runs over when you gun it, the head, or the headgasket, is history, Or the block or head may be cracked. Take it to your mechanic for a free evaluation and an estimate. You may be better off getting another car and trading that one in, and let the dealer deal with it. They can afford it.