could be a bad card or bad power supply. start with opening the system up and making sure all the fans are moving..if there moving slow or sluggish..may be time to replace them. make sure you run a program like asus afterburner set a fan profile of 50c make sure the fan on the gpu maxed out and watch the gpu temp. if the temps get to hot the gpu will lock up. on the power supply side run hardware monitor and log the ps voltage..when the system locks up look to see if the voltage were fine in the system.