'02 Cavalier- car idle surging w/AC on and trans in drive

Hi- my 02 cavalier always idled smoothly with the air conditioning on, now it surges from 600 rpm to about 1000 rpm, constantly while at a stop sign, and you can hear the AC relay clicking on and off. I need to either shut off the ac or put the engine in park to stop this from happening. It also seems to occur well after the engine has warmed up. Thanks for any help! -Tom