Shoutbox

MTU is not likely to be the problem here.  It will be one of the two: intentional filtering (which some ISPs offer as an option) or routing issues at your ISP or its upstream providers.  The latter would include the potential problem mentioned in the quote in your last post.

There are tests we could go through to pinpoint which it is (and, for the routing issue, where), but neither will be repairable by doing things to your computer; it will take contacting the ISP anyway, so you might just want to start with that.

I doubt it is filtering, because you are also having problems with CNN.  While I could see a "safe surfing" filter give a bad rating to some forum, it is not likely they'd blacklist a major news site.  However, you can ask if they offer any filtering and if it is enabled for your account when you call. 

So, call and complain that you can't access www.cnn.com and some other sites.  Focus on CNN rather than your forums, as that is something other people are likely to complain about as well.