Vigor2962 keeps dropping off the LAN

I have a Vigor2962, connected to a 1Gbs leased-line.
All 20 LAN profiles are live, as well as the routed subnet.
Site is a serviced office, and can go for 12 days without mishap, and then the router drops off the LAN completely.
I have a PC running at the site, running logged ping-tests to all of the other LAN hardware - mostly Netgear GS managed switches - and everything else stays up without a hitch.
There are typically up to 150 devices connected across all of the VLANs at any one time.
Can anyone help with this, please?