Wont take off when at temperature and the air conditioner is on

I recently had the radiator, thermostat and water pump replaced. Now, when the car gets heated on hot days, (not overheated), and the air conditioner is on, the car wont take off. The rpm's will go up but the car does not move. If I turn the air conditioner off, it will then accelerate, but sluggishly. Also, when this happens, the fuel hand will drop. It will eventually rise back up. Lastly, the car is using an extremely large amount of gas. Like getting eight miles to the gallon. What is all of this about?