Sporadic Idle With A/c On

I have a 99 Mustang V-6 5 speed. With no a/c the idle is steady at 800. When a/c is turned on idle goes to 1700-2000 and then fluctuates from 900 to 2000 RPM. Need Help ASAP

Check The O-2 Sensors. And If You Took The Cats Off It Will Defantly Cause This Problem. If You Took The Cats Off You'll Have To Put Them Back On Or Reprogram The Computer. Goodluck, Hope This Helps.