The new Graphics card may have agrevated it more, as magiK said it is probably related to heat,try putting a new fan in or take the side of the case and blow one of the big fans in if it still crashes my dad had a problem like that he found out that the memory in the slot was slipping out of place causing the computer to crash,or your system might have a conflict.if non of the above help then go to the microsoft website (or search on google)and type in the error you will probably find the source of the problem.look on the bright side you have an error to search for witch is more than my dad had.
Hope you work your problems out
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