I live in a capital city with a very high speed internet connection and with recent computers/devices, so this is clearly not the issue here.

Likewise. Using gig speed internet with brand new router on various latest generation devices. The devices/connection handle other sites and applications that are much more image and data heavy. Don't think it's poor device/connection speed.

How long after the page loads does it jump? And which pages does it happen on? Is it more noticeable on long threads?

It happens to me when I start at the last post of a thread and then try to scroll up. A few seconds after scrolling up, it jumps back down to the last post. It doesn't seem like it matters how long after the page loads I start scrolling. Then I scroll back up again, and when going from the last page to the second to last page (or any page n to n-1), there's another jump - this time jumping seemingly randomly further upthread.

I'm going to jump in here in favour of the admins. It's bugging me too, but it's like that on many ad-funded websites. It's down to the advertisers, and probably to some responsive stylesheets. Because not all ads are the same size, you can't predict the size. So when it is loaded, the browser will adapt and you'll get that jump. My 2 cents. The only way out of that is to have a subscription-based version of the site where there are no ads, but that wouldn't be everyone's cup of tea.

same for me. Overall it lasts a few seconds, but it's just so annoying on a tablet.