1993 Mustang Cobra irratic idle. Normal?

My 1993 Cobra changes idle frequently. When it's just sitting there running, it will idle where it should. Then there's a click and it idles around 800-1000 for a bit then drops back down. It may have always done this, but with my new louder exhaust it is very noticable. I've tried turning my A/C off to see if it was the A/C compressor clicking on, but it didn't change it. It sounds like a compressor clicking on under the hood and revs a bit higher, then clicks and goes back down. It does this repeatedly. Any help is greatly appreciated.