More info is coming... this time from a Dutch forum where I'm also a member of tweakers.net and Mr-Beamer did a very interesting thermal test on XFX 7900GT...


Image of the GT.. top of the image is the Zalman 9500.. on the right observer the white spot..


Enlarged image of the white spot... white represents heat which is over 80°C as you can see..


Regular shot of the problem area and you'll see the ISL6549 chip which produces the problem..

Googling for the chip gives us the info that the chip is rated up to 70°C and not the 80.8°C measured..
So in short... the back end card is overheating and this proves it.