Any Toyota Techs on here?

I have 99 4Runner that has me stumped. I have problems with the heat. I have replaced the thermostat, heater control valve and heater core. I have checked the blend doors. It comes up to temp fine, does not overheat and, its not losing coolant. It blows hot at higher RPMs or while driving. Come to a stop and blows luke warm the same at idle. The only thing left is the waterpump. Just not convinced that its the waterpump. Any help would be appreciated.