I’m with @japes. Any ads with sidebars are interfering with the browser scrollbar/page-down key. A page refresh always loads a new ad. So if the new ad has no sidebars, you’re good. If the previous ad is reloaded or it’s another with sidebars, you’re stuffed.
I keep on refreshing until I get an ad with no sidebars. Or if a sidebar ad has block-booked DIS for a day, I use the thread scrollbar to go the latest reply and then use the browser scrollbar or page-up key to go back to the top. So making the entire thread is visible.