Hello,
We have iMacs at multiple sites that go gray suddenly during use. There are no foreign letters -- just gray.
The only way to restart it is to unplug the power. This started happening about a month or so ago. Have you experienced this? Seems to only be iMacs, not any other models. I checked the polices and everything seems to come down okay.
iMacs are on 10.9.3
21.5- inch, Mid 2010