I've noticed the same things my end.
But it seams restricted to certian servers. The main one I can think of is with the Dell website. I can browse the main website fine, but as soon as I click on Configure and access configure.euro.dell.com I don't get anything. Firefox can be sitting there for two hours and still be waiting for information!
Today I've been browsing the Apple website - apple.com is fine, but apple.com/uk is a no-go! Rackservers.com is a waste of time, but
http://www.rackservers.com/configurator.asp will load the header and footer frames instantly, and I'll be stuck their waiting for main content.
Google, BBC, Dilbert, Gentoo, Suprnova.org, Demoniod, PC Pro, Amazon are all fine though. I've tried with and without proxy servers.
What sites are you having trouble with - is it the same ones as me?