Happens to me all the time. Its really bad when its my first playthrough (FFX and XenoSaga 1 are still waiting for me). And my latest playthroughs of Xenogears and Beyond Good and Evil bottomed out too, they're not such a big deal, because I still love them and already know what happens anyway. Though I too have no clue why motivation drops out like that. Even now I think about those games, and kinda want to play, and just don't. But in my case I think I'm just different. I've sorta dropped out of being a gamer all together. It takes a really rare gem to capture my interest enough to complete a game. Ones that have already been beaten sorta default to the back of my collection.