2006 Opel Unknown problem

Over the past 3 weeks my car has gone in 3 to 4 times. Everytime I get the car back the dealer says the problem's fixed, but it isn't.
This is the problem:
Whenever I switch on the air con there is a stranger whirring sound, the car won't pull; it drives sower no matter how much gas you give, and you can absolutely forget about getting up a hill when the air con is on. This problem only happens at the hottest tie of the day. Lunch time. It doesn't happen in the morning or when it isn't hot.

The dealer says it's normal for the air con to make a sound when it is on. Obviously. However this sound is different to the normal sound. These problems weren't there before and I'm at my wits end.

Please assist asap

Thanx