gt2003mustang

my 03 gt runs excellent my ac works great but when i run the ac or defrost everything is fine at first but after about ten minutes when i come to a stop my rpms will stick high about 3000 and it will stay that way until i turn off the ac or defrost and it will take a little while before the rpms are back to normal i know when the ac or defrost is on the compressor stays engaged any ideas would be helpful thanks