Sounds like you should get your Refrigerant checked and topped up, Too little refrigerant can cause this problem. You have to get a A/C workshop to check this out and refill.
Thank you Jonny.
Other things to check....
Fans. Make sure they are engaging.
Coolant temps. At low speeds the engine may get hot and automatically cut off the AC. There is a AC cutoff sensor in the radiator. Regardless of the AC refigerant, it wont hurt to pay some attention to the cooling system
Balance shaft pulley. If this is broken, it will have the same effect.
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