Hi there! First post as i was googling around the 7970 Matrix Platinum for answers of my own to some GPU Tweak issues and i stumbled across this thread and thought i'd give you my experience.
First of all let me start my telling you i owned the GTX580 Matrix Platinum and moved on the the 7970 Matrix Platinum on release (pre-ordered it), so I've had a lot of experience and have been bombarding ROG with feedback.
Here's a few things:
Matrix Platinum 7970 like the GTX580 version, uses a bespoke (as such) voltage controller. Which means that GPUz can't detect any changes, i set to 1.35v and it will still say 1.256v. I would use a multimeter if you want accurate results or use GPU Tweak monitor for software results.
Likewise, no other overclocking software will work with these cards due to the voltage controller and sometimes the fan controller too. So you're stuck with GPU Tweak if you want to control everything.
90c+? I've NEVER seen that temperature at 100% fan speed and i've pushed 1.36v through this card. I assume BOTH fans are working?
Related to fans, please note that since the later versions of GPU Tweak, the fans work independently to each other. Fan1 is linked to the GPU Temperature and Fan2 is linked to the VDDC (VRM) temperature.
There is a firmware update to this card. You can download from ASUS' website, it may or may not help in this issue, but what have you got to lose?
If i were you, I would test temperatures outside of Kombustor, it's extremely taxing and unrealistic. Run Heaven with the settings below over 5 loops and take the highest from that.
http://farm8.staticflickr.com/7197/6860929862_c30aaa43a8_b.jpg
If you still see 90c~ then i would start an RMA process going if i were you tbh. Something is definitely up with your card.
If you have any further issues then i am over on the
Overclockers UK forum and on the
ROG forums. Though i'll check back here at a later date.