Your old GTX 670 with 4GB performs better than your GTX 980 with 4GB.
Your old GTX 670 with 4GB performs better than your GTX 980 with 4GB.
You're facing some performance issues when switching from your GTX 980 to your i5-3570k. The new GPU struggles more with modern games, especially at higher settings, which is why you noticed lower framerates compared to your older model. It sounds like the differences in GPU power and architecture are affecting how smoothly the games run. You might want to check your drivers, consider using a better driver or software, or explore options for upgrading your graphics card if possible. Your warranty is still active, so you have some time to resolve this.
It seems you want to uninstall your current GPU driver, restart your system, and then set up the 980 driver.
Check the temperatures first. Consider removing the existing drivers using Display Driver Uninstaller and then installing the latest versions.