[ubuntu] can't connect to or even PING certain sites

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http://ubuntuforums.org – Hi all. I've run into a problem I just can't figure out how to fix and was hoping someone else might be able to help. Running karmic, it seems my computer just CANNOT connect to certain websites sometimes, even though those sites are not down. I mean I cannot even ping the site. When I try to ping, the DNS routes just fine and I get the IP address with no problem, it just never gets any response. Trying to load the site in any browser just hangs and finally returns "unable to connect". Things I've tried: disabling ipv6 via the about:config page (no effect) adjusting MTU on my links (HowTos)