Sorry you are having these jumping issues - we also run a fairly large board, and have not had any complaints about similar issues.
The post jumping works fine - and it would be nice if it were improved to have a link to the last unread post at the top of each thread (in addition to the current method)
The only way that my users would jump to the most recent post, if if they were to click on the newest posts table in the sidebar -a part from this it works as intended.
Have you got your users to clear their cookies if they are having serious issues - might help
