The A/C clutch will engage even when defrost is on. I would get the car to operating temperature, open the hood and have someone turn A/C or defrost on, you'll be able to hear the clutch engage with a click usually and if thats when your problem occurs I would suspect the compressor. It will only be available as a complete unit and you'll need to recover and recharge the freon when it's being replaced. Another way to do it would be to unplug the compressor connector so it never engages and drive around, if you still have the problem then it's not the compressor.
08 G6 3.5L Sedan - Daily Driver
97 Eagle Talon TSi AWD - Weekend Warrior