Reinstalled PC with Win 10, getting FPS drops during gameplay

Hey there,

I did a complete wipe on my PC and installed Windows 10 Pro, along with installing the latest drivers for my hardware before I did get FF XIV downloaded and started with DX11.

However, with my old System I never had problems in the game, but right now I receive massive FPS drops in areas with reflective surfaces like water and other elements. On Hullbreaker Island, the second and last boss causes my FPS to drop under 15 FPS.

Specs
Windows 10 Pro, x64 System
AMD Phenom II X4 945 Processor 3.00 GHz
8GB Ram
AMD Radeon HD 7800 Series, 1GB - Using 15.11 AMD Catalyst with 'Performance' settings.

My temporary workaround now is to switch from Windowed Borderless to Fullscreen and back, but only works as long as I stay in the same zone.

Any idea how to fix that?
Thanks!