2001 BMW 330 Air conditioner works intermittently

My air conditioner seems to have a mind of its own. When it works, it works fine. Most of the time it does not work unless I am on the freeway. However, even on the freeway it works 50% of the time. On occasion, it will come on when I start my car, but will only stay on for a few minutes. Also, seems like it works more often when I roll my windows down (maybe just a coincidence). It seems to work more often when I turn my interior thermostat up past 85 degrees (also maybe just a coincidence). Any ideas? Thanks