air conitioning sometimes works sometimes does not

Sounds like a slow system leak. You can have the system charged and pressure checked for leaks. If you are a handy sort, you can spend you weekend hunting for a potential leak. Fill a generic spray bottle with dish soap and water and spray the lines going to/from your compressor and the components in the loop (Thermostatic Control Valve & Condenser/cooler). Easiest thing to do is to take it to a local shop.