Shoud8375

I have lexuses350 2008. I am experiencing some problems with my ac system. Ac works perfectly fine when I set the temperature at lowest. Then it blows nice cold air. But when I set temperature at 22 or 23 degree Celsius then it blows warm air and gradually hot air. The outside temperature is 41 degree C. Then if I set the temperature again at lowest, after a minute it blows cold air. Don't know what could be the problem. I have visited my local mechanic. He says heater core and valve needs to be changed. Any good suggestions on that??