Hmm, never used D-Link or Buffalo myself. If you have the option of taking the D-Link back, I would recommend trying a Linksys BEFSR41 - the entry level 4 port non-wireless Linksys product. Just over £30 on Amazon.
If returning it is not an option at this stage, you could check the logs on the CM (see
http://homepage.ntlworld.com/robin.d.h.walker/cmtips/ for details) and the router, but if a single PC works fine, as does a hub, I can't see any reason that a router shouldn't work OK.
HTH