Rendering with 980ti x 2 stuck at first two tiles

Hi,

I recently upgraded my GPUs to two 980 ti with sli. This is my first experience with multiple GPU. My problem is this; I successfully rendered two architectural visualisation projects, encountered no problem, performance was good. Today I was rendering a scene and render crashed somewhere in the middle with Unknown CUDA error. Nvidia drivers also stopped and screen gone black for about two second.

After this happened I wasnt able to render with two GPUs anymore. Render starts and first two tiles appear, it seems as if clearing samples is going on but very very slow. When I try separately each GPU, it renders fine.

I tried a simple scene with cube and sun (500*500 resolution and 300 sampling), it managed to render the scene with Double GPU. But when I added HDRI to scene, it did the same thing, Just stuck at first two tiles. And again rendering separately with each GPU worked fine, but when I try both it gets stuck.

Any ideas why this possibly can be happening?

My system;

Two GTX 980 ti
i7 5820K
32 GB Ram
Windows 7

blender 2.77

If you are using multiple GPUs then you should not be using SLI