The comments in this thread mention things like that.

http://www.tomshardware.com/forum/40403-63-suddenly-connect-internet

See christian06 and leftbrainfemale's comments near the end.

It used to be I had all kinds of trouble with networking Winders until I put everything on static IPs. I haven't worried about that too much with 7, but most of our stuff is still on static IPs.

Another thing to try is a different DNS. Google's are usually pretty reliable, if not the absolute fastest.

8.8.8.8
8.8.4.4

I'm assuming you're not using IPv6. If IPv6 isn't configured properly, some sites will disappear as far as your browser is concerned.

HTH a little. Good luck. Let us know what fixes it.

Cheers,
Scott.