Dell laptop intermittently freezes with a checkered pattern (horizontal and vertical lines) on the screen, after reboots it works fine again

My laptop randomly and intermittently freezes and the screen shows horizontal and vertical lines in a checkered board pattern. I cannot associate it with heavy load or anything in particular, it happens very randomly. The only fix it is to press the power button fora couple of seconds to reboot the machine, and after that it works fine again, until next time it freezes, which can be in the next 10 mins, or after a week. Sometimes it freezes multiple times in a day ( 3x). I have windows 7 and ubuntu installed, and it happens with either OS. I have run memtest and other tests and nothing comes u

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