Ati 4870 artifacts and freeze in Games

Hello guys im a new one here to the community.I came here because im struggling with crashes when i play games.When i play games like Black Ops 2 or Skyrim,Mass Effect etc my computer crashes at the exact same screens and artifacts appear and then boom black screen so i have to reset from tower.For example in Mass effect 2 everytime i was on the galaxy map-->boom crash in Black Ops 2 most of times in create a class screen --->boom crash.Some other times crashes come randomly but most of times i know where they are going to happen.I would like to let you know that im running the latest graphic card's driver amd catalyst 12.6.So here is my system and is a bit old like 4 years ago or something:
Motherboard:Asus P5Q
Inter core 2 quad 2.6ghz
Saphire ati radeon 4870 512 MB
OS:Windows 7 64bit

PS:I have tried to clean the case with compressed air but the problem still exists