ND5 XS possibly playing up

For the last few weeks I've been having some issues with my ND5 XS :-(

After faultless operation for several months I'm starting to get something strange happening. It will just stop playing part way through a track. When this happens the display starts flashing network down and going back to the UPnP server screen. After a few mins it'll settle down and I can reselect the album and start again but, once is starts playing up like this, it'll repeat the fault a few times before finally playing a full album okay. I've changed everything I can on the network side: New switch. New LAN cables. Full reset on the NAS. Reinstalled the media server software. Even reformatted the NAS and reloaded the music library. When this is happening, if I look at the media server GUI it says 'streaming' so the media server thinks it's still playing. Note I'm getting into the NAS GUI find whilst this is going on which leads me to conclude that the network and NAS is fine.

The problem is it'll go several days okay and then start playing up again so not easy to debug :-(

The only thing I can think of doing now is to replace the NAS but that's a little extreme when I don't even know if it's the nas that's to blame :-(

Has anyone seen this sort of issue before?

Thanks.