iTunes stuttering and stopping

I think I've fixed this problem. I moved the iTunes library files (.itl, .xml etc) to the internal drive on my iMac, leaving the media files on the NAS. I then renamed the .xml file and moved it to another location, launch iTunes, create a new playlist (blank), quit and relaunch. iTunes rebuilds the xml file and the whole thing starts working again with no stuttering or stopping. Still the occasional second or two of beachball when accessing a new album or playlist, but playback is seamless and continuous. However I lost all my artwork... tried copying over the artwork file but to no avail - no art. Still I can live with that now I at least have reliable music streaming.
I don't think this issue is with the NAS / disks / firmware but is something iTunes 12 'broke'. Last night I watched a hi-res movie ripped from a BluRay on the highest profile HandBrake conversion settings, on a WDTV, and the NAS streamed it perfectly. Bit rates would be far in excess of that required for iTunes.