You don't have some exotic rotation system set up with your wallpaper do you? If not your system resources may be low. Right click the taskbar and click task manager to see how much
memory you have free when this occurs.
You could lock the wallpaper in, if you click here and save it to desktop then double click it, it will merge a file to your reg that prevents the wallpaper changing. Just click here and download and do as above to reverse the clampdown.