Have you added any electonics? Use your cell phone (charger) in the car? Nextel is bad for causing interference. Other than that no real idea. That's what your dealer is for. Take it to the service dept. and hopefully they can duplicate your issue, unlike the problem I'm having with XM.:hmm:
Look at the top edge of your front and back glass on the inside.
Make sure all the buttons are still attached to the windshield and back window.
Next, there is a little door on your rear center dome light, open it and loosen the bolt you see, then retighten it.
Could be a bad ground for your antenna module.
If that doesn't fix it, take it to the dealership, the antenna module could be bad or not plugged in properly. But if you have FM it is probably still hooked up, just messed up.