I personally am not having this problem (in any of the browsers I've tried--FF, Chrome, IE8), but quite a few of the members have mentioned it since the 5.0 update. I thought it was the page trying to finish loading and asked them to be patient.
"I'll be reading something then off it jumps back to the top of the page. Sometimes I grab the scroll thing w/my mouse and hold it so that I can get through a reading. Usually after about the 3rd attempt, it will stay where I want it to without much ado."
I did a search and could not come up with anyone else mentioning it here. Have any of you heard or had this problem.
I asked for browser and browser versions and am waiting for replies.
If your site takes a long time to load - or has a lot toad AFTER the forum is displayed then this could just be a result of that - especially on any url that has the post number at the end as it will scroll to that automatically. If the person started to read from the beginning and had already scrolled the page but it was still loading content in the background then you would see a jump when it finished loading. Not really sure you can do much about that but you can ask the users in question if the loading is still going on when they see this behaviour.