A/C does not blow

You have a leak in the system and are short on refrigerant, when at high speeds the air going over the condenser is cooling the refrigerant and slow speeds its not. Get the car up to temperature and then with the car running turn on the A/C, lift the hood and keep your eye on the AC compressor, you should hear the AC coming on and going off fast most of the times within about 5 to 10 seconds...if it's going on and off fast then you need to add refrigerant to the system...