I would trust your diagnosis first. Usually if the compressor works at times, it's working, ... Period. There ARE things that can cause them to work intermittently, but that is rare, and it won't be so consistent as what you have observed.
Your description suggests there's a problem with air flow through the condenser which is right in front of the radiator. There will be an electric fan that should be running when the AC is turned on. If it's not running, there won't be any air flow at low speeds and that will prevent the condenser from giving up its heat to the surrounding air. When the refrigerant stays hot, it can't absorb any more heat in the dash where it needs to start out cold.
Saturday, July 11th, 2015 AT 9:04 AM