Another thing to check is if there is a newer firmware version available for your Motherboard BIOS.
Something similar happened to me about a year ago, when my ATI Radeon seemingly just shut down at random occasions. Turned out that my graphics card was too new for the firmware my motherboard shipped with to handle properly. After I updated the motherboard firmware to a newer version that problem has never occured again.
Since you upgraded from an older card to a newer one something similar could be the cause in your case.