Thought I would log this, just in case anyone else sees it...
I was seeing tracks come up when on random that wouldn't play - the track information was shown, but no sound (or sound was delayed). I thought it was the connection, or firmware playing up. Turns out its a new (to me anyway) feature of the iPhone that your library now also includes "cloud" tracks - i.e. ones that are included in my music library at home but not loaded on your iPhone. The option is in Setting | Music | Show All Music; I've shut off the feature now....and situation resolved.
I was seeing tracks come up when on random that wouldn't play - the track information was shown, but no sound (or sound was delayed). I thought it was the connection, or firmware playing up. Turns out its a new (to me anyway) feature of the iPhone that your library now also includes "cloud" tracks - i.e. ones that are included in my music library at home but not loaded on your iPhone. The option is in Setting | Music | Show All Music; I've shut off the feature now....and situation resolved.