Ah, the attack of the plot bunnies! It never fails - they always swarm when you're in the middle of another perfectly good book.
Like you, I make notes about the new story. I will, though, then go back to what I was originally working on. As long as I've captured the essence of the idea, I know I can always go back to the new story when I finish the one I'm currently working on. The problem, though, becomes which of those stories do I work on first when I finish my current WIP - decisions, decisions!