Coloured And Dotted Lines Across My Screen

Hi, I have got an NVIDIA GeForce 6200 graphics card. Now theres somthing wrong with my graphics card which is causing dotted lines to go across my screen. These lines are are all mixed up rainbow colours and in the bios screen they are just black and white dotted lines. Now I had this problem checked out by my cousin who is a computer technition and he said it was the motherboard that was playing up on me. So my cousin got me a new motherboard but we still had the same problem as before. So my cousin said that there was somthing wrong with my graphics card and told me that I needed to buy a new one.

But Im guessing that maybe my graphics card is over heating. Because the problem happens only during the hot wheather, but during winter I didn't get the lines across my screen. But now that summer is returning here in australia and the wheather is getting hot, Iv gotten the lines across my screen again!

Also when I try to instal the driver for my graphics card it starts installing to 50 percent, then my computer restarts and it comes up with send error report, somthing wrong with NVIDIA Graphics Card or somthing like that.

What do you think the problem is with my graphics card?