Did you recently upgrade the video card without also looking into whether the Powersupply could handle the extra power the new PSU required? -- usually shutting down like that under load is a sign that the system is not getting enough power to remain stable. What Powersupply and system specs are you running? Or it could also be a temperature problem - Have you checked the temps of the system when the shut downs are happening ? if temp related make sure the fans are all working properly and you have cleaned the dust etc. from the system as it can cause heat buildup.
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Are they pirated or genuine? because I might know the reason.
If it's not software based, overheating is a possibility.