[WARN] Thu Apr 21 17:43:05 2011 Failed configuration authentication attempt by IP address 192.168.0.195
Nahh, this has nothing to do with internet access. You get this when you try to sign into the router's access page and type in the wrong password. Was just able to replicate and got the entry above. Not to say your not having a issue with that computer getting online, I just don't think that log entry has anything to do with that issue.
As for your issue, when you loose the connection on that 1 PC, do you lose it on all of them? When you say "re-boot of MAC" do you mean the Cable modem? Is that 1 PC wired or wireless?