This kind of "CUDA error 999 on device 0: unknown error " are usually related to hardware or drivers issues. If you are using risers or external boxes try to render your scene disabling the "use for tonemap" option in the plugin options panel, GPUs tab.
Perhaps even one of the GPUs is physically damaged. Test the GPUs one by one to see if you can isolate one of them with problems. If you are using the latest NVIDIA drivers, perhaps a good test could be also downgrading a couple of versions to see if this helps.
Thanks,
-Juanjo