It's very hard to say what's wrong without a lot more information. It sounds like either your computer is overheating or your graphics driver is crashing.
If you have a look in the Event Log, you might be able to see some information about what the computer was doing when it crashed.
http://windows.microsoft.com/en-gb/windows7/open-event-viewerIf you can tell us a lot more information (what kind of computer is it? Does this happen with anything else? Can you play normal DayZ online? etc) then someone might be able to pin it down a bit more.
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