Unable to connect to certain websites

I have a bit of weird problem that I've been noticing for some time now and it's really starting to annoy me.

For some reason, I am unable to visit certain websites. When I try to visit them in my browser (any browser) I get a 'can't connect to server' error message. If I try to ping the website in question, I get a 'request timeout for icmp_' message in the Terminal.

This is only the minority of websites. Most sites work fine. There seems to be no link between the websites that do and don't work. All devices on my home network fail to connect to these sites. Before you ask, they aren't dodgy websites (one of them is www.petsathome.com). I know the sites are up because if I visit them on my iPhone with 3G they work fine.

I have turned my firewall off on my router. I have tried using Google's DNS instead of my ISP's (O2 home broadband).

Any idea what might be the issue?

Thanks,

MadDoc