FPS drop in same Area of Orb Valis

[Avalgilda]

Hi DE I have a some problem in game. it's only happen some area in orb Valis map. 

When i'm step in the Map my FPS was so smooth I'm locked(Vsync on) my FPS at 75 same as my Mornitor can do. but when i have to go inside a Cave(not all cave) a FPS was drop to 57-59 FPS sometime drop to 41-43 but never below then this  but screen was not smooth when FPS drop

and some Cave there i saw a kind of strange Polygon was popped up and disappeared in a sec.(like an Environment : rock,stone,root tree,grass etc) it's not every cave will happen only 2 or 3 cave. and it's not about a Cave's size some are smaller cave still get a FPS drop too. another Cave are good to go smooth FPS no sight of FPS drop only few place.

I've try all Low setting and Epic setting it's the same i really don't know how to fix it.(already Verifled and Optimized too) 

my PC spce 
CPU ; i5-3450
Ram : 6GB
Graphic : GTX-1060 3g 
Monitor : Old Samsung LED with VGA - HDMI from 3 or 4 years ago 

Thanks for anyhelp 

Edited November 19, 2018 by Avalgilda