display screen shifts to left and right

Hi, The display on my new iMac will occasionally shift about 1 to 2 inches to the left or to the right. It will hold that position for several seconds. During this time the system is unresponsive to the keyboard or mouse entries. If the screen first shifted left it will then shift right, still off center but in the other direction. The screen will then re-center itself and responsiveness returns. This may occur 4 or 5 times during the course of an evening. The problem was worse when I primarily used Safari as the browser. It happens less with Foxfire.

Suggestions or answers?

Thanks,

Fred