Sounds like something I recently experienced. It sounds like the ski snaps around as you move forward over the front in the offside? If this is the case, you probably need to reduce length as you said.
When I was experiencing the same thing I felt like I was either riding the tail (no turn initiation) or it would suddenly snap around on me with no happy medium. Reducing length resolved this for me.
.005 would be a small, incremental move. .010 would be a good, noticeable move. .015 is a large move that would dramatically change the turning characteristics. Based on what you said, I'd make a .010 decrease in length and then fine tune from there.