Rainbow six siege vsync problem on Fullscreen

[sgtbreezy93]

Hi there! 

Straight to the point. Whenever i play Rainbow six siege on fullscreen with Vsync the fps will be 60 fps then drop to 30 fps or 34-35 if 75hz and then its go back up. playing on borderless windows solves the problem but i want to play on fullscreen (why? Takes long time to explain it so i wont do it) with Vsync. I can play tf2 on fullscreen with vsync without this issue and many other games too (Like CoD BO3, BF4 etc..). Also dota 2 has the same issue.

CPU: FX 8320 (stock)
GPU: R9 380 2GB XFX
PSU: corsair cx750m 
Mobo: Asus M5a97 R2.0 
Monitor: AOC G2460VQ6 (24 inch)