You have flashed PC version of the X1900...ever thought that could be the problem??
Well part of the problem is I have it tuned to full speed (650/700 core/mem).
Other than that, no chance. A GPU should not be able to heat itself into artifacts unless overclocked or improperly ventilated.
Keep in mind I have it setup to put the GPU fan on max when it hits 60C.
I was going to replace the cooler with a Zalman uber cooler but it didn't fit in the mac pro's case in slot 1. My friend bought it off me for almost what I paid so I'm not that pissed.
I was going to buy another cooler but instead I'm waiting for card updates and listening to my lawnmower in the meantime.
Oh, also, a LOT of Apple's (detuned, pre-flashed) X1900 cards have been sent back to because of quality control problems. Apple knows this product is crap, which is why I'm sure they're working hard to remove it from the product line (probably have an outstanding contract with ATI/Foxconn).