I have a client with a fairly new Windows 7 Professional computer who, several times a day, experiences a freeze-up of his system for about 30 seconds. Afterwards, it starts working again. When it freezes up, the windows vanish, and in their place, these diagonal lined window outlines appear in their place. I've not been able to reproduce the behavior, but today, he sent me a photo. What causes this behavior? See photo attached. Thanks.