There is a great book by Richard Ferber called "Solve Your Child's Sleep problems" - available at most libraries. We've experienced LOTS of sleep issues with our 3 boys, including with our oldest who is normally a GREAT sleeper. I've looked at lots of books about sleep issues and talked with our pediatrician - they all come back to Ferber. He gives various scenarios - you'll likely find something similar to what you're experiencing, and then explains how to fix it. His book says that if followed, sleep issues will almost always disappear within 3 weeks if not before - in our experience, it's usually been only a few days to a week when we're back to "normal" after following the advice given as "prescribed."