FPS drop

[BravoLupin]

As title, my PC spec is :
i5 4690
8GB ram
GTX 970
Current i'm running the game at 1600x900 and middle setting (everything is at 3, vsync, AA off)
at that setting, i have around more than 120 FPS normal, more than 60 FPS when raid
But sometime in raid, i keep getting FPS drop down under 20FPS then go back 60+FPS, making my game lag and unplayable
And sometime, even freeze game for 1-2 sec (0-1FPS)
Can anyone help me with ? :(
My english is bad, hope u understand, tks