Looking into this a little more there are a couple of things that might be wrong here.
If I have got hold of the correct manual for your Graphics Card (
http://www.gigabyte.com/products/product-page.aspx?pid=2996#ov ) it states that a: Minimum 680 W power supply specification and with both an 8-pin and a 6-pin PCI Express supplementary power connector is recommended.
Your PSU is rated at 650W and although it has been working OK, overtime the efficiency and power output can drop off. It might be that, when you are playing some of the Graphics Intensive Games that he PSU can't provide enough consistent power. It might be worthwhile trying a different, more powerful PSU if you can, or getting your one tested to make sure it is still delivering enough power.
The second thing is that your Card has developed a fault and the only way of being sure about that would be to try and test it in another PC if possible.
The third could be that there is a problem elsewhere in your PC and the best way to check that out would be to replace your GTX 295 with another graphics card and see if the problem still occurs.
This doesn't cover all the possibilities of what could be going wrong but they are the areas I would be looking at first in trying to solve the issue.