I have a similar problem on one of two D620s I own. But it can't be heat related because it happens right after login with either the Windows XP or Windows 10 Preview (10121) hard drive. It is related to memory modules. With 2 GB bottom only it never happens. With an additional 1 GB on top it happens now and then. With the full 4 GB it happens every boot up. But always at the exact same point in the boot process. And it's just as if you had no battery and pulled the plug on AC adapter. That sudden. I suppose I could try a different cpu, sine I have one lying around.