Well, (fingers-crossed) I think I've solved it - after many months of trying.
A post on another forum suggested restoring the router to it's factory defaults. I have a DLINK DSL-2640T. It seems that over time the firmware within it becomes corrupted. There is an option to save the current configuration (which I did), then restore to factory defaults, then reload that saved configuration.
Not only can I get thru' the options of the router quicker than recent times, but after streaming for around 5 hours I've not had the problem re-occur.
Additionally, before this fix, certain videos which had problems playing were 18mins 18secs long according to the WDTV display, whereas they were actually much longer. This has also been fixed.
So there you go, anyone with a problem like mine, try this.