Cooling Fault

[mictop2003]

New on here so just getting used to but I have a R56 Mini Cooper with cooling issues, If you start off in the car the heaters warm up and stay hot but after a while they go stone cold and the car overheats, Last night I drove the car on the motorway with no problems until I got stuck in traffic same fault heater went cold and the car overheated. I have changed the following parts all to no avail:

Thermostat Housing (Twice)
Cooling Pump & Intermediate pulley

I have bled the system but the fault still occurs

Any Ideas

Thanks