What really screws up the compatibility is the agp/pci-express bridge chip these cards have. Having the gpu at a 45 degree angle makes few after market coolers work.
My card kept locking with the stock cooler and heatsink. The heatsink on the bridge chip had an improperly seated thermal pad that seemed to be the reason for the instability. I ended up replacing the pad with thermal grease, and the card stopped locking up.