IE & FF “shake” when in Full Screen mode.

I have a client whose computer screen will shake when Internet Explorer or Firefox is in fullscreen. By shake I mean, the browser window is changing from full screen to maximised continuously resizing the page. Happens when using the latest version of FF and IE7. PC is running Windows XP Pro, only other software installed on the machine is MYOB and Office 2007. This glitch doesn’t happen all the time, they can be using the computer for days before it will happen, it seems to mostly happen on the ninemsn.com.au page. I have reset all IE settings, re-installed FF, problem still occurs. When it does happen, pressing F11 does nothing, have to use Task Manager to kill the IE/FF process.

Any ideas?