I had a similar problem to this once and it turned out to be a loose wire connector to the clutch on the ac compressor. There's a wire going to the compressor that powers the magnetic clutch. If you remove the compressor, you can unplug the wire. However that plug may get dirty and worn and break the connection and reconnect several times in a hour. I cut it out and butt spliced the two wires and that was the end of it. Start the car and turn the ac on. Lift the hood and watch the compressor to see that it is spinning ( not just the pulley but the whole clutch ) and not turning on and off erratically. Also check the pressostat which is mounted on the reciever/dryer. It may sense low pressure and shut the system off and then change it's mind and turn it back on. It's cheap to replace but if you are borderline pressure it may be trying to tell you something.