Why does my heat blow cold air on a 60 degree day w/o overheating?

What is your question?

The heat blew cold air even while moving for a long time then randomly started blowing slightly warmer air and proceeded to get cold again. It was about 60 degrees outside when this happened last. The car does NOT ever overheat. I have replaced the water pump about a year ago. The fluids were flushed then. I put a new thermostat in just before getting that work done. I know that the problem can be the Heater Core or an air lock. What else can cause this? I dont want to put more money into solutions that are not helping.