Update....
Apologies for the delay in responding: I went on an e-Bay Auction Watch to source a used replacement DS. I picked up a nice and clean DS213+ at a good price so all good on that front.
I have migrated my drives into the DS213+ and have full access. Migration was an absolute breeze. Accordingly, much of this data has now gone through 5 migrations as follows:
Initial migration from a Windows NT Server to a Lacie NAS
then from Lacie NAS to DS207+
then from DS207+ to DS209
then from DS 209 to DS212
then from DS212 to DS213+
The reason I'm making this point is to confirm that I have NEVER had any difficulty migration forward on Diskstations. Every time, I get a quick, clean result which speaks volumes (no pun intended ;-) ) for the DSM Interface and underlying OS.
As to the specifics of that migration, I purchased a Belkin USB WiFi stick to accompany my 'new' NAS as a small and cheap piece of fault tolerance on the LAN front. I stuck religiously to the Synology Compatibility pages, having previously tried to get an unlisted SMC stick working to no avail.
As to the question of backups, I fully agree with stevech and sfx2000 regarding the need for backups of a NAS. However, my data is relatively static and, while I had a full off-site backup of 99.99% of my data, that last 0.01%was relatively recent and had not yet filtered to a backup on my current plan. I am now up to date.
As to the originally posted issue, this remains. I am fully satisfied that the problem exists on the NAS- not cables, nor router nor LAN port on other devices. There is no network flooding going on- the problem is in the NAS. I hope to get to the bottom of it now that the criticality of the DS212 has diminished. As the problem is intermittent, I still suspect a motherboard problem such as as a dry joint, but I will nuke the current DSM and install DSM 6 as soon as I get access to it next.
Finally, a big shout-out and thank you to stevech who offered me a 'recovery' of some data if all else failed. Much appreciated..