When I turn on my ac the fan instantly comes on, and the car shudders like the ac is taking all the power?

It also blows really cold unless it's real real hot… If it's real hot it tries to blow cold for awhile… Then starts blowing warm air. If I turn it off for a while, it will start blowing cold air again… For a minute. (I live in Florida)
2003 Ford focus
125000 miles
Repairs… Serpentine belt, alternator several times, water pump, and a few other things.
It started after I took my car in to the shop to have my alternator/serpentine belt replaced.

When you turn on a load such as the headlights or AC the Idle speed control motor is supposed to adjust the idle up to handle the extra power draw on the motor. You need to have the engine data read to see if all the sensors and solenoids are working properly. Go to the dealer.

Why did you not return to the shop that did that last repair? Seems logical to go back to a shop that did something that affected the car. No one can "fix" anything online by guessing, even experts admit to that and it would be irresponsible to say otherwise. Cars are very complex machines that wear out, get damaged by use and weather and elements, etc. Asking here is asking for guesses. Besides you are not a mechanic. Right? Go back to that shop.