Jerky replay - new problem, same equipment

I have a Node 2i connected directly (ethernet) to my router, which is connected directly to the NAS with all the music. Lately the music has become very 'jerky', especially when playing higher resolution files. But (a) this is new - none of the connections or equipment has changed; and (b) even at 24/192 you only need around 0.6Mb/s and I easily get 10Mb/s to and from the NAS. I've checked for updates to both NAS and Node, and rebooted both.