my ipod classic will not sync with itunes

I have recently upgraded to OSX Yosemite 10.10.5 and iTunes 12.2.2.25. Initially I 'lost' all my music in iTunes and when I finally managed to get this back, nearly 50% of my CD artwork had disappeared. I have now uploaded some new CDs to iTunes and created new Playlists only to find my iPod Classic (Software version 2.0.4) will no longer sync with iTunes (I do have plenty of spare capacity!). I've tried switching off my Classic and re-syncing but am getting the messages 'Connected' for 10 seconds, 'Synchronising' for 3 seconds, then 'Ejecting' followed by 'OK to Disconnect'. Does anyone have a solution to this extremely annoying and time wasting problem?