Never had it (knock on wood) and here's why: whenever I feel I might be about to get stuck, I pull back and think of the engine that drives the story. The ETDTS is not the plot or the hook or anything like that. It's really about the spirit of the book, actually something missing in more than a few novels. The ETDTS in the Chet and Bernie books is the love between Chet and Bernie. When I remember that, some narrative route always suggests itself.