Problem with 8800GT in 08 Mac Pro

I'll try to make the background on this issue as short as possible while still giving all the relevant information. 3 weeks ago my Mac Pro began randomly freezing. Apple's Hardware Test, Memory Tests, etc etc all came back good. After a lot of troubleshooting between me and the local Apple store I have resolved that the issue is my 8800GT overheating. The card is fine running my two displays normally with photoshop, browsers, email etc. If I load up iTunes and set the visualizer to full screen the card will freeze up in 30 seconds or so. If I crank my IO and exhaust fans way up I can get the card to hang in for another minute or so. Today I removed the stock heatsink on the card, cleaned the old thermal paste, and installed a fresh coat of thermal grease. This did not fix the issue.

So right now I'm debating biting the bullet and buying an overpriced Radeon 4870 from Apple.... or buying an aftermarket cooler for my 8800GT. Zalman's VF900 seems like it would fit my given card.

I've noticed that the fan on the video card itself does not seem to change speeds at all despite the load on the card. I was under the impression that the fan should adapt to the gpu's load/heat output. What would cause the card to run hot after a year? I ran WoW a lot on it the past year and never had any trouble at all. The short version of my question is.... is my video card fried? Or do I have a cooling problem resulting from fan speed that can be fixed with an aftermarket cooler?

 

ideas anyone? Last night I installed Windows 7 on Bootcamp, installed the latest Nvidia drivers and cranked up RivaTuner. I was able to specifically find out that the video card glitches at 67 degrees Celsius. When I crank the fan up to 95% this takes about a minute to happen, with the fan on its normal 29% setting this takes about 2-3 seconds. I'm going to research more, but I'm pretty sure the card should still be working at this temp.