Ken,
I narrowed down the problem to the length of the menubar versus the top of the footer. If the top of the footer is = to or above the bottom of the menubar, then the background changes depending on where I put my mouse or scroll the page. This seems to only happen on the wide screen setting. As long as the top of the footer is below the last thing in the menubar, everything works fine. I noticed this because only the pages that were short had the problems. I added some <br/> in the main body of the short pages to make them longer to fix the problem for now. I will do some more snooping around in a while.