Assassin's Creed 3 Graphic bug

I started the game a month ago, but didn't manage to find the time to play past the opera scene. So far all was working well. THis July when i wanted to continue my game, the menu started to be messed up, kinda trembling. When i loaded my last save, the character is not walking properly. In the captain's room, I'm only pressing W to go forward, but it's like there are obstacles I can't see and can't go around. after 15 minutes i finally managed to get out, but I've found no way to fix this. I checked the GPU's drivers, reinstalled the game through steam,, nothing yet....