I think, after doing a water pump and thermostat, I would look at the cooling system being plugged up or see if you have a head gasket pushing the coolant out of the system.
Need more info.
How long does it take for the system to overheat? Are you losing coolant? Have you felt the heater hoses going to the heater core to see if they are getting hot? Is your electric fan coming on? Does the car overheat while driving down the road or more when you come to stops? How hot does it get? Does the heater have any heat coming out or is it just not hot enough? Will the heater get hotter if you keep the engine rev'ed up?
what made you decide to do the water pump? was it leaking? Was there a coolant loss before? Is there a coolant loss now? After the engine seems to overheat have you shut the engine down and felt the radiator to see if it's hot too? (feel the radiator core to see if it has cold spots...be careful so you don't get burned if it is hot)
Heater cores are just mini-radiators. they can plug up and now allow flow through. you could try disconnecting the heater hoses and see if you can either blow low pressure air or water through to see if it's plugged up. (low as in under 20psi)