Man... when it idles back and forth like that, it's getting the wrong signal. It sounds to me like you swapped your IAT sensor harness and your IACV harness. They have the same plugs. It causes exactly that, but usually you get a check engine light with a code 14.

Something tells me you've done more maintenance to this car than you're telling us. But if it just started doing all this by itself, I've heard of stranger things happening. I know someone that did the thing I listed above to someone else as a practical joke. Wasn't very funny.