Memory overheating on 9800 pro? Chip temps are fine

[Uberapan]

My goal is to build a high performance fanless computer. I have succeeded, if it wasn't for the video card.

I have a Radeon 9800 Pro, and I watercool it with a Zalman Reserator 1. However, something else is overheating on the card. Without fans blowing on the card, it can't even complete a single 3dmark03 run. After a minute or so, the screen becomes corrupted and the computer hard locks.

This leads me to believe it is a video memory problem. The memory heatsinks become very hot to the touch. The chip is cool though, since it is watercooled. If I add a fan blowing onto the card, everything works fine.

I don't quite understand how it can be overheating, since I run the computer with the side case panel off at the moment. Case temperature is therefore quite low.

The reason I'm asking is that I've seen reviews of the Zalman fanless TNN case used with my video card, and also with a X800 Pro. That apparently worked fine, and the case temps in those reviews were at least 10-15 degrees C higher than mine. That case also has zero airflow. How is this possible, if I can't get it to work with an open case?

Any ideas?