I get this syndrome rarely, when I'm working through something that I'm sort of nonplussed with (I only read one at a time, and have never been one to stop reading once I start).

Recently I read The Other Emily (Koontz, because it was free on Amazon and I thought, hey, sometimes I like his books). By about midpoint >!(when it kind of jumps genres about four times)!< I started thinking, "well, this isn't outright horrible, but I'd sure rather be reading ... something else."

In this case, the something else was Children of Time (Tchaikovsky) and let me tell you what. Not thinking I'd rather be reading something else.