I was playing around with this yesterday and here's a couple things I found , I was having similar issues as you and also like you I've moved up from a 158 to a 162.
Stance width. The narrower I went the more washout I had on my back foot. The wider i went the less this was a problem. The next width wider was just to wide and wore my outer quads out to quickly. 23 1/2 vs. 22. Being as the wider width was too wide I started to work on the washout issue of the narrower stance.
As Sno mentioned above, I ducked out my back foot and it really helped keep my upper body more in line with the board. I'm now at +15/-15, I was at +15/-3.
This led to another problem and that is initiating a turn. I felt all messed up while cruising the blues. Damn near was catching edges. I can't remember where I read this, probably here. I put my hands behind my back and that's where they stayed no matter how much I wanted to move them. This solved my turn initiation problem. What I suspect is that at one point I got in the habit of leaning my body weight back, as well as open stance. Whatever it was, hands behind my back forced a proper body position and as a result the best feeling I had all day.
Hope this helps....