Your new monitor has a much higher resolution (2,073,600 pixels compared to 1,296,000 previously). It means your GPU has to work harder to render each frame. You can drop settings, or change from your native resolution (to make the game render frames the same size as before, instead of a new higher resolution), but your best bet is to just drop some settings.
Drivers can be another issue obviously. guru3d.com has a great utility called DDU (Display Driver Uninstaller) and you might want to use that before reinstalling your new drivers. I just google search "DDU" and the top one is the link I click, but google does search tailoring, so it might not be top for you. Good luck.
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Aside from trix4rix suggestions, re your install prob (there are some reports of auto installer failing), try a Clean Install AMD Graphics Drivers
Do not use the "auto...." or "min setup" for this.
Problem solved, I just installed Windows 10 and all my problems were fixed. It even runs better than previously on Windows 8.1 Pro