Car shakes...

...when I have her in park and she's idling. It only happens when I turn everything else off (radio, A/C, etc.). It's like she's trying to keep from stalling out and the bouncing effect created by that makes my car shake and my engine makes a bouncy noise (the rising and dropping in rpms). It wavers around 500 rpms...sometimes it'll drop down to around 250, then kinda go up to about 600 rpm or so. It'll do this bounce in rpms around every second unless I turn the A/C on or something. Anyone have any ideas what's causing this or what I could do????

[Mr. Pockets]

The number one cause for a rough idle is a vacuum leak. You may have a cracked hose or connector. Take a good look around, and you may even be able to hear it.