I can't pinpoint the problem with my bike, but the feeling is that it suddenly feels like I have no chains, and then it comes back, like it's "skipping".
UPDATE: From the comments, I think the proper description is that it suddenly stops engaging from the freewheel. Usually it happens when I'm vigorously pedalling, or when I pedal in reverse. There's also a snapping sound when it happens. And then it returns to normal after a while.