I've had this issue with 2 cards. first card was only 1.5 gb second card had 4gb. It would seem to be a memory issue. It doesn't have to be a huge scene for it to happen. Just needs to be a scene that was running for a while and rendered a few times. Memory seems to leak over time and a simple save and restart of DS fixes the issue but it will eventually happen again. I run GPU-Z often to check my temps, memory, and GPU statuses. When the scene has been active for a while I'll notice that the GPU no longer will be active and it will resort to CPU rendering. This is a lot slower of course. The card with 1.5 GB would throw back to CPU more often due to memory limitations and eventually lead to a crash. The 4GB card is a bit more hardy but it too will hit that wall and resort to CPU rendering. A reboot of DS often does the trick and I can get a large scene to render through GPU once again.