Why would the ac fan works some of the time?.

Does your vehicle have "automatic temperature control" -- as noted by a digital temperature display? If so, there is a blower speed controller that works with the temperature control to regulate fan speed. If not, there would be a conventional relay and resistor for the blower motor. Diagnostic testing is needed to find out what is going on with your blower motor inputs and operation. www.repairpal.com/repair-shops