I have a similar issue. Also on Windows 8 Consumer preview. When i first open iTunes it will play just fine. But if i change playlist or double click on a different song it wont play the song for about 5 mins and then plays it. Exiting and opening iTunes again allows me to pick a new playlist and play it instantly, but if i try and change song by double clicking again it stalls again.
I'm in a similar situation, using Windows 8 Consumer Preview with iTunes not playing songs when double-clicked on. The way I get it to work is open any explorer folder and double click on another folder or click on a folder on the left sidebar. Music always starts to play immediately, and the glitch doesn't come back for a while.