Weird FPS lag when using borderless mode

[Avere]

I am having a weird type of issue i have not seen in any game other than ARR, When i play the game my FPS is a solid 60 but at times could happen recently but it feels like my fps has dropped to 20 and that all of a sudden goes back to smooth frame rate this happens off and on i really don't know what the problem is, i have reformated my pc 2 times already and even reinstalled the game countless of times to try to fix the issue and it's still their.. i have checked my temps on both graphics card and processor everything is in the green.. so if anyone can help me that would be awesome thanks. ^_^

Intel core i7 920 4.0ghz
Geforece GTX 760 2gb