Why does my 2014 Nissan Sentra have random fan speed

Hello, You might have a charging system issue, slipping belt, or bad A/C compressor clutch that is slipping when you request more power from the engine. Issues with the refrigerant system are also likely, but these should have been found when the recharge was completed (Assuming the shop provided you with a certified A/C repair technician). Have it diagnosed, and don't let the shop sell you on evacuating and recharging. That will not help the situation unless done as part of a repair for the actual issue.