Done some digging around, seems there are a few people having the same problem. The first solution was a cable problem and the fact that the card was slightly loose in the PCIe socket, so if giving your GPU some extra juice still doesn't work then i would recommend opening up the tower, removing the GPU then putting it back firmly into place. Check to power cables, make sure they you are using a full 12v rail (i assume you are because it has worked fine up to now).
Also have you overclocked your CPU recently? after reading around it would seem that an unstable CPU clock can cause GPU problems when in game. Try resetting the clocks back to stock settings if you have overclocked.
Still nothing, try looking at this video http://www.youtube.com/watch?v=ClxEtUxl6AI
- the solution was to get a new monitor (extreme i know, but if you have another display, you could test your card and see if its your monitor)
you could also try following this guide: http://www.youtube.com/watch?v=AAHc7Ha3y0E
If still nothing then really would recommend you take it to a PC shop. I would avoid any chains stores as they are stupidly overpriced.
Could also be a bad HDMI/DVI cable....