Specifications:
Intel H55 Mobo
4x 1GB Corsair DDR3-1333 ValueRam
Intel i5 750 CPU with Zalman CNPS5 CPU cooler
Leadtek GTS 250 512MB
Windows 7 ULTIMATE 64bit
Gigabyte 585watt (Model: GE-585A-C2)

Load temperatures:
CPU: 45 Celsius
Graphics 58 Celsius

Problem:
In games: Screen starts to go black after a while and then flashes back to game... Keeps on doing this more frequently and for longer if you keep on playing. When you COME back there is normally a graphics glitch like a small line of blurred graphics for a second and then it goes back to normal. This happens more frequently in graphics INTENSIVE games.
[Happens the quickest in Racing game: GRID]

What I've tried:
*Installed new CPU cooler(ZALMAN mentioned above), CPU temp was running bit high.
*Dusted components/ fans
*CHECKED all the connections.
*Installed latest drivers for graphics card (from leadtek and then tried latest from nvidia.)
*Uninstalled Intel TurboBoost
*Did windows updates

I suspect that it is my graphics card. I'm considering taking it to the place I bought it and asking them to take a look at the card since it is still under warranty.

Any other suggestions/ Advice??
If it is still under warranty i woulld not HESITATE to return it for a replacement...