I dunno... I don't seem to have that issue. Could it be browser related?
Also, I take a different approach as well. I open a 'New Posts' tab and read from there. Why hunt around when the software does it for you?
I doubt it's browser related, I visit a few other vbulletin forums and don't have the same problem. I doubt many others use the same forum reading technique I use, open the forums list, open each forum in a new tab and view the list of posts, and open posts to read in a new tab.
I've been reading forums like this since before there were forums and usenet was the only way to get help with cygwin/perl/iis/cgi problems, and I like it. I've tried reading just new posts, but I enjoy refreshing my memory from viewing the overall lists of forums & posts. Often an older thread catches my eye and viewing just new posts I wouldn't see it.
Kind of like how I don't like documentation without a table of contents.
Back when I worked at CDC as a hardware guy and got sent to a training class on some new hardware, the first thing I'd always do was quickly scan the table of contents for all of the 50 lbs of documentation dumped on the table. Modern search is great for quickly finding an answer to a specific topic, but just searching for the topic at hand isn't particularly educational in "the big picture".
Anyway, if you use just the new posts list you'll probably never see my issue. I believe it's a php session timeout setting, not a vb or browser setting.