Air conditioner problem. The mechanic cannot fix unless they can duplicate problem. The compressor is charged and has no leaks. It will either freeze you out, or it is warmer than air outside. Yesterday it was putting out hot air, so I lifted the hood and the compressor was turning. The mechanic showed me how to check. On the average it happens about once a week. I have tried turning air conditioner button on and off, turning off engine and starting again, switching all the selection buttons on air console. Drove it three times yesterday between 6: 30 a.M. And 1: 00 p.M. And it was freezing. Drove it three times between 4: 00 pm and 9: 00 p.M. Yesterday and got hot air. I drove it around midnight and it was freezing. What are the possibilities of things that could be wrong with the system?