Firefox and IE won't occasionaly connect to popular websites

Hi
I have a very strange problem.
Occasionally websites like cnn.com, msn.com.au, google.com, yahoo etc doesn't load in both firefox and IE. Or websites load absolutely fine eg. whirlpool, businesspectator etc. That after a while could be 30 minutes or so and it will start working again. Restarting the computer does not solve the problem. I have a second computer on the network and that computer is able to excess those websites fine even when the problem computer can't so I guess the issue is specific to this computer in question. I know this sounds very strange and I am at my wits ends trying to solve the problem.

Someone suggested that I change in the MTU in the router to 1500 done that but still no luck.
I tried to ping those websites when it is not loading up it won't go through.
I have discovered if I type in the IP address for google into firefox I can go to the home page but I am unable to navigate any further i.e searches don't go through.
I have run various malware removals:
Superantispyware, adware, anti-malware,combofix, spybot
I have run anitvirus:
trendmicro housecall, avast.
I had macfee security center running initially I have uninstalled but no luck.
I have run TCP optimizer.

Sorry for the long post but could anyone please give tips as to what we can try next.

 

If you have a wireless card attempt to connect to someone's network other than yours when the problem occurs. Attempt to load the website this will eliminate hopefully one piece of equipment. If it goes through you can safely say it is not your box, of course make sure wireless on your network fails to connect. If it does not work you can narrow it down to your networking equipment.

I have heard claims that ISP have intentionally blocked certain web traffic for example comcast and torrent sites, hopefully this is not the problem. I know my ISP frequently blocks my access to TPB. I can connect via a dsl connection whereas I cannot connect via comcast.

GL