It *is* the UBR - see the thread triggered by my similar query a while back:
http://www.cableforum.co.uk/board/sh...ad.php?t=18194
My router was reporting DOS warnings triggered by the UBR - the advice was to configure the firewall to let these through, but since my router's firewall won't let me do this (Belkin), the upshot was as long as things were working OK it was alright to ignore the false alarms being triggered ...