Upon joining a friend's server in story mode, I came across myself doing less and less damage right outside of Startington. It seems to only happen when I swap between 1H and 2H a lot. It doesn't happen to them, and I haven't seen anyone else do Titan Throw / Piercing Dash build, so I throw my 2 hander, swap to 1H to penetrate through the enemies to pick up my 2H again, and proceed to rinse and repeat. (It doesn't happen immediately, but after a good 10 - 15 minutes it comes out of no where) The only fix I have seen is to leave his server and rejoin. Ah, and it only happens in Story Mode.