So...random persistent page loading problems.

[Merlock]

So I keep randomly (not too often, but still) getting this odd problem that I've seen attributed to viruses or adware, however, as I've made as certain as one can be that my system is clean, I figured I'd post this here just out of curiosity of whether others have experienced this. At random times while browsing online, Firefox (I'm using the latest 3.5.1 but this has been happening back in 3.0 too) starts loading pages but freezes up with only a small part of the page's content displayed and just stands there supposedly... Waiting for pagead2.googlesyndication.com... And it's that same link it loads every time this happens, so I'm guessing it might be a problem with ads rather than locally? Because it's only sites that load the above link that end up not loading fully (YouTube among them). Others are fine during such "fits". And more so, these "fits" last for quite some time and nothing I've tried locally seems to fix them until they vanish on their own. So the question is...has anyone else ever encountered this?