I got a brain teaser for ya. Why does my aux heat come on when it gets hot out. At night when the outside temps drop, the unit works fine. By 10 am, it will throw on the aux heat. Some times its at the start of the cycle, sometimes its during. Regardless, its not cooling when it does. Its been high 90's here daytime. What I've done so far is to turn off the unit at the breaker, then turning it back on a few seconds later. It seems to kick start the cooling, but its not a permanent fix. By the next cycle the problems start again. The filter is clean, the coils are clean, the unit outside is clear of debris. I believe the unit is less than 8-9 years old. I live in a 1400 sq ft modular. Outside unit and inside unit are only like 8 feet apart. Could it be my thermostat (old looking mercury switch type). Please help, mother coming to visit soon, and long summer ahead.