2003 Honda Civic Air Conditioner Intermittent

I have 1 2003 honda civic LX sedan. Starting at the end of summeer 2009 the ac began to blow warm air. A leak check was conducted but no leak could be found under the hood. I was told the leak could be behind the firewall. Spring 2010 we start the AC up and discover it is blowing warm air. Another leak check, no leak found under hood so we have freon added to get cold air. About 6 weeks later the ac started to work intermittently. It would blow cold air like an AC should then the next thing you know it would completely shut down. Whenever the AC completely stopped you could turn the key and hear what appears to be blowing then it shuts down. Turn AC off then back on. Nothing. Not even a sound. Turn key off then back on; you hear blowing sound again. During this time no air is coming from the vents. Hours or days could go by and the AC would just start working. We were told the control unit was bad. Had it replaced Friday. AC working find. Saturday night, AC completely stops again. As of today the AC is still inoperable. Help! What is happening. Any explanation why or recommendation of what the real problem may be with the AC? What would cause the AC to start and stop?