Go through your Installations Log in System Report and see if any updates were applied last month. I had one come in for my MacBook that changed that build number and then was causing me issues with sleep behavior. I ended up having to reload the OS to clear it up since Apple pulled that update off their site after 48 hours due to issues with it. This may not be your problem, because that update was specific to laptops from them, but it's possible that they released another update similar to it that was built for your type of system. Checking certainly wouldn't hurt.
