This is a problem that has plagued my office for a few months now. It has bounced between our ISP (Comcast), our IT contractor, and even our VOIP contractor, each of which has "fixed" it at least once.
Originally, the problem was significantly worse than it is now, with sites constantly timing out of half their assets: Loading the DOM but only some of the images, or timing out on the CSS. That problem has become much rarer, but still, many times over the course of a day, a domain will simply fail to resolve after a wait.
Suspecting Comcast's DNS, I tried switching my workstation to Google DNS, and then OpenDNS, neither of which changed things: Now I'll just get the OpenDNS page telling me it can't resolve google.com.
And that's the part that perplexes me -- how can this problem transcend DNS providers?