i have 2 LS170. one is fine and that's why i know the other could have an issue. the drive controls are very jumpy, jerky. each time you start off it wants to buck jump unless it's at very low rpm. hoping there's some kind of adjustment, etc.
It sounds like your steering linkages are loose somewhere between the sticks and the pump. Any play or wear can cause it to jump around. My S250 was like this when the arms on the pump were worn, you'd try and move slowly then all of a sudden it would jump forward. I removed the play and it works great.
