A/C sometimes won't start?

[Greedo51]

Sometimes when I start my yaris the A/C will not come on. This happens about once a month. It began occurring about 12 months ago. Some times the A/C will kick on minutes after starting the car, sometimes hours, sometimes the next day. I believe it also happened with the defrost in the winter. I made an appointment with my local toyota dealer when it last went out on me. The next day when I had the appointment the A/C started working. They checked it out and could find nothing wrong. They said it needed to be not working for the computer test to show an error. They said if it goes out again to rush it back up there and they would check for free. Every time it goes out I head towards the dealership, but the A/C comes back on before I get there. Does anyone have any idea what the problem could be??? I would GREATLY appreciate any insight.