Just got the same problem even I am opening multiple threads too. I think you will have to make posts too not just surfing. JUST AN IDEA
Just in case you didn't notice, your total logged time is listed in the left top corner of the page, like this:
Otherwise, i think a click every fifteen mins or so (refresh or new topic) is enough to keep the timer going, so if you're reading some long wordy topic, click the refresh button