I was talking to a writer friend of mine over the weekend, and our conversation inevitably turned to those sticky moments in a story when you know that what you're writing might not be entirely believable to the reader, but you really need the plot to go in a certain direction, so you hope your reader will suspend disbelief long enough for you to get there.
Sometimes it works; sometimes readers aren't that patient. I took an online workshop over the summer that addressed that problem in a great way, though: make sure you have a minor character who works as the "skeptic" in your story.
Simply put, have a minor character voice the doubts you know your reader will be having at any given point. Know your main character shouldn't fly to Iceland after his estranged girlfriend without a plan? Have his best friend tell him all the reasons he's being an idiot for getting on that plane. Know your reader will wonder why the heroine is letting a stranger into her apartment when three other women in the apartment complex have been killed? Have her mother tell her so over the phone (right before calling the police because her daughter's clearly lost all rational thought).
Those skeptical comments from other characters in the story will signal to your reader that you as the author are still in control of this story. You've put your characters in questionable situations, yes, BUT never fear: all will turn out well, and you know exactly where you're going with your plot.
I really connected with this piece of advice when I heard it; it made a lot of sense. And it's already saved me with a couple of plot problems in my current WIP. Try it; add it to your arsenal for those times you write yourself into a situation that seems questionable even to you ;)