For a while I have been having a strange problem with Amarok. You could set up a playlist, let it play the first song and then wait upto five minutes before it moved to the next track.
Anyway, enough was enough tonight, (a guy needs Dave Grohl to run to!) I found this helpful little post on the Arch forums. Basically it is a known problem and if you switch to xine as your backend for phonon your good to go.