Q: Screen Freezes/Shifts

Using Yosemite, Safari the screen freezes when ever I zoom in on a page. The cursor will move but I get the spinning wheel. They only I can get out is to click on an empty area of the screen and then close Safari. This has only occurred on 2 iMACs but not a MBP running build 10.10.2 or a MBA running 10.10.1. It also does not happen when using Firefox. I also have screen shifting even when not in zoom mode.