If you look at it, there is a lot of problems related with GTX 970. Some are related to NVIDIA drivers, some users even downgraded it to get the performance back. Also there is a problem related on GTX 970 and 980 when the VRAM memory usage goes above the 3.5gb ( Why Nvidia's GTX 970 slows down when using more than 3.5GB VRAM | PC Gamer )
If that was the case, try to make some tests. Change the preview quality to one that uses less memory and check if the FPS normalizes.
I know that might be a shot in the dark, but since, no one commented about it, I believe that might be something related.