Have you checked the condition of your waterpump, it could be that the pump is not functioning to its full potensial and when heavy load is put on the engine e.g. when going up a hill, the reduced flow of water is not sufficient to cool the engine down.

I will wage my 5c ont the waterpump, there is only a limited number of things that can llead to a over heating engine:
1. blocked inner radiator core
2. blocked outer core of radiator, air not getting by the fins.
3. none functioning thermostat
4. blown head gasket
5. water pump

In your case i wouls suggest cleaning the radiator, get a new cap, if this does not solve your problem, change the waterpump