What make/model PSU? They are not all the same. Install Speedfan or similar to monitor GPU temp. Check GPU cooling fan is spinning. Speedfan can also chart th value of 12V rail. If your PSU is good quality, 12V rail stays constant and GPU temp stays under 100C, you have faulty video card, return it under warranty.
Then it's either the PSU or the graphics card. Put Speedfan on Chart tab and display the 12V rail. If the 12V stays flat while stressing the card, get it replaced under warranty, it sounds like faulty memory on the card.