I've got more data.
First, the problem seems to happen a lot more with Firefox than with Chrome. I'm not sure if I've had it happen on Chrome at all, in fact, since I don't use it as much.
But most importantly, I seem to have found EXACTLY where it happens: the eBay Free To Air Equipment Finds forum listing is where the page stops loading. So, I removed that forum from my preferences (I never used it anymore anyway), and I've been problem-free since!
To clarify, it's not IN the forum itself (at least I don't think so, I didn't enter it to see), it's the listing for the forum in the top-level forums page.
I'll leave it up to the management to figure out what the problem is beyond that. I can say that I DO see other references to pixel.quantserve.com being loaded successfully, it's just whatever is attached to the eBay FTA forum that won't load.