As mentioned, you probably have to turn it off for each sound. I've
had problems with the velocity on a small midi keyboard, after
pouring coffee over it (who would have thunk that). It was full
velocity on any key stroke on two keys. Besides that, those keys
lagged terribly, too. I got it almost fixed by cleaning it
thoroughly, opening it and detaching the keys and the soft rubber
bumpers on the board underneath the keys. Maybe cleaning everything
possible could fix your after-touch, too.