In start of a multiplayer game for example escape from stratis my FPS is solid 60 and dosen't drop from that.

After around +1,5 hours into game play FPS start to drop untill the game becomes unplayable

I have seen this problem in few games before and in thoses it has been memory leak.
I can still play the game and i have enjoyed it alot. However longer missions are hard to play due to this problem