FPS drop for unknow reason

[xenal]

Like it says in the title, my fps randomly drop by half for no reasons at all, until I close the game and come back. It can occur mutiple time, so I can drop to 15, then 10 then I never passed it. The time of it occuring is random, often happen in arena. No this is not my graphic card glitching, no this is not due to high graphics in certain area. Being an assassin at 30 fps make pvp impossible.