I'm sorry if bumping topics in this forum is not allowed, but I have to in order to address this issue.
Galaky, I am having the exact same problem, and my PC is basically the same as yours. I've tried memtest86 and it didn't give me any errors, my motherboard (Dell 4700) is not supported by Motherboard Monitor, and my fans aren't detected in Speedfan either. I have been using an external fan blowing on the card but it doesn't seem to have any effect. At first it did stop the crashing, but it has seemed to go back to the way it was before I tried using the fan.
I would appreciate a solution. I do not want to have to resort to buying another graphics card.