Well, we developed a BAD crash bug in the program. It took the programming staff several months to narrow it down to the real problem. After going through several of our own systems, writing new memory management, and other fixes, we finally discovered the real problem. There is a bug in the Nvidia drivers that doesn't work properly with D3D 9. We've contacted Nvidia about this issue, but gotten zero response. We looked at other released games, and they crash with this bug as well. The publishers of those games must have just given up and let their game go out with the occasional crash.
In order to fix this problem, since Nvidia isn't going to take care of it, we have to write all our own code to handle error checking issues with D3D. This will take us considerable time and effort, but in the end we will no longer be dependent on the Nvidia code to keep our game solid.