there could be a leak in your condenser. You might try to find a mechanic that could either check the freon level or add dye to the system. That way if there is a leak an area will be colored with UV dye or whatever color he uses. We've had to do that once or twice.

I know when we check for a leak we put a vaccum on it and see if it holds for awhile or see if we can get it down to 20 or close to 30. I know ac checks if you can find them will only cost you maybe 30-50 dollars depending on where you go. Try that if you keep hitting road blocks.