Sorry for the late reply guys, I've been doing lots of changes with my rig, made a clean installation of windows and also played with the voltage settings of the graphics card, so now the thing is that, artifacts seem to be gone when I underclocked the card a lot, the settings I have now are:
Core Clock: 500 MHz
Memory Clock: 650 Mhz
Core voltage: 1V
Memory voltage 1.5V
Performance is not that good but at least games are playable, do you guys have any suggestions about what should I do to get a higher and stable clock speed??