Save your game[to a new file] then restart the ps3
You probably don't even need to restart just quit the game, when you load back up the game will be running smooth again. It may be slow going to save that game but spending five minutes getting it to save is better then the alternative.
I few other tips, don't overwrite save files, always save to a new file and later delete the old ones manually, this makes the game more stable in the long term. If you have the classic pack, dump your canteen don't ask me why the game just runs better for me at least if I don't have it in my inventory.