The a/c compressors pulley is on the timing belt, so when you turn on the a/c, the compressor turns on and takes some horse power and gas. My car does the same thing but I just rev it and it stops sputtering. When your car sputters does it sound like its about to die and shake the car? I'm not a licensed mechanic but I how it works. Give me the year, make and model of your car and ill ask a few mechanics.