This is a pretty regular occurence with me. Probably daily if not multiple times a day. I am a huge fan of all kinds of music though, so the variety is ususally pretty diverse. I remember one time many years ago I was very sick with a high fever and there was a song on the radio while I was laying in bed. I spent the next two days in a fever induced haze repeating the same two lines from the song over and over in my head. Pretty nasty stuff. I still remember it, it was "Bring on the Rain" by Jo Dee Mesina. I am huge U2 fan so I can personally think of worse things to have stuck in your head :)