Usually when I hear the term Ear Worm being used, it refers to the songs that have at least two qualities: they are easy to get running through ones head, and they tend to be very annoying to listen to. While I've had this happen, I have to admit that more often than not what I have running through my head is not a song known as an "Ear Worm." It usually is something I've heard recently. In some cases its something that I haven't heard in a while, but am reminded of.
Also, a lot of the time I will have a decent chunk of the song running through my head. But there are sometimes that the jukebox in my head gets stuck. This can, on occasion, be more annoying than an annoying song. The problem is that this seems to happen at really bad points musically. I'll be hearing over and over most of a phrase, but not the final resolution, which can get very frustrating.
As an aside, I'm reminded of a story my high school band director once told. As I recall the story was about a hotel with a piano in lobby. Someone went down and played "The Star Spangled Banner," but stopped before playing the last chord and then walked off. Not that long later, there was someone else at the piano trying to find that last chord to resolve the tension that was still hanging around.
I've had similar happen to me within my own head.
Fortunately, I know the cure: listen to music -- either the song running through my head or something else.