by topping off the rad while it is running? Is the top (upper) rad hose getting warm after a few minutes indicating that the thermostat is opening? Do you have your heater turned on and set to max heat while you're purging your colant system? Are you using a 50/50 mix of coolant? When you replaced the waterpump and thermostat, did you use oem? Are you sure you didn't miss-install the thermostat? Are you waiting till the thermostat is fully open and the upper rad hose is hot before replacing the rad cap? Are you filling the reservoir to the correct Full mark?