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.