I've recently had the same problem. AC/heater worked fine all winter, but then, as temperatures climbed, the AC started acting up.

I think my problem also was in the ignition switch. Today, I sprayed some electrical contact cleaner in the key hole, and followed that up with compressed air. Lots of black stuff came out of the key hole. I also thoroughly cleaned the key.

AC now comes on regularly when it's supposed to. My fingers are crossed.

You might want to try the electrical contact cleaner. I don't think it can hurt anything. It's available at Auto Zone, etc., stores.

Good luck.