For a long time, my second monitor in the dual display mode would revert to the right side, and not stay put either on top or the left. After much research and frustration, I stumbled on the answer. Right click on a blank screen, and click "align to grid". I am not altogether sure why it works, it just does. My theory is any custom setting confuses the graphics card and it returns to default (right side for # 2).
Spike's advice: Backup your data routinely.