Lee,
UDP port 68 is the bootp port, so it's 'possible' that someone has attached a pc to your local UBR thats trying to find a bootp server. (Could happen if a hard drive has failed and the PC is allowed to boot on lan)
There was an old virus/worm that used UDP 68 (see
http://www.us-cert.gov/current/services_ports.html) but i would think it's unlikely to be this.
I doubt whether this would cause the router to fail, but you could check with Buffalo's knowledge base to see if there are problems with bootp.
HTH