Why does screen shift sideways without warning?

Since getting a new iMac with Lion, I have noticed that frequently the screen will shift sideways without warning, when the mouse has moved only slightly, and disappear. Sometimes there is an arrow in the lower righthand corner that allows retrieval of the lost screen, but not always. This causes particular problems when my son is doing schoolwork online -- sometimes the answers he had put down to questions will be gone when we retrieve the screen, or answers to matching-type questions will be scrambled. Is there a way to stop the screen shifting sideways?