OS X :: Graphics Card Not Picked Up By Hardware Test / Parts Of Screen Starts Going Black

I've got a 2007 Macbook Pro. It's on its second graphics card (8600M GT), the first one failed on me a year ago. OS X 10.5.7, all firmware updates installed.

I think I have a new graphics card issue, but I can't prove it. In Front Row, after 10-20 minutes, often parts of the screen will start going black. Then it freezes. Sound works fine. It doesn't look like the same kind of wild distortion that I got when my GPU failed the first time, though, it just looks like things have been rendered incorrectly.

When Front Row finally quits, I've had black and white bars across my menu bar a couple of times. Once the computer then totally locked up and I had to reboot.

In Windows XP in boot camp, things are worse. I only use boot camp for games, since they use a lot of graphical power, it made sense that the problems happen there more often. After 15-20 minutes, the screen will start stuttering, then the keyboard/mouse will stop responding. The sound will hiccup a few times too. That lasts about 10 seconds, then everything returns to normal for a while (as little as 10 seconds, or as long as 5 minutes.)

The Apple Hardware Test (short and extended) shows no trouble. Techtool Pro shows no trouble. I ran the very thorough memtest86+, and it showed no memory trouble.

Can anybody give me any tips on definitively diagnosing this problem? Is there any kind of really intensive hardware test I can get (a live CD maybe)?

Second question, has anybody heard of Apple replacing a laptop that has had many faults with a brand new unit?