This is from one of those flashes of intuition that, far too often, turns out to be way off target and totally useless. So my apologies if that's the case here, but I thought it might be worth checking into.
Since your problem seems to be associated with "large and complex" sites, by any wild chance is/are your browser(s) exhibiting this problem capable of (and set up for) pre-fetching multiple links it finds on a page? I doubt if this would affect modern systems much, since they're much faster (and have far more resources) than older-generation systems. One way to check this might be if you can find a "site map" page, which should (theoretically) give you a worst-case scenario situation because of the multitude of links there.
