Why are websites selectively loading for me?

I’ve noticed that websites are selectively loading for me both at home and at work. Some sites will load quickly, some won’t even resolve at all and it seems to be pretty random. For a while Facebook wouldn’t load (so I know its not a case that some or other website is just offline) and today its some other site.

I’ve tried different browsers so its not a browser specific issue. This issue also started around the same time that our ADSL connections became pretty dodgy (slow speeds, erratic throughput rates) so I am wondering if that plays a role. Even if that did, it would have to be a general network issue because my office is a few kilometers away from my home.

Another possibility is that my DNS settings need to be changed. Those are common to both my routers and are set to Afrihost’s DNS servers. That makes a bit more sense, actually. Perhaps the solution is to change my DNS settings to OpenDNS’ or Google’s DNS servers. Any other ideas?