Does your machine have a cruise setting? I have a lot less problems when I keep mine on about 6%, which means when I stop, the machine keeps stitching slowly. It allows for taking an extra stitch at points and stuff like that.
You mentioned this is happening on the back of the quilt, so I assume the extra white thread is the TOP thread, and not the bobbin. If that is the case, is that caused by backlash? I've had the opposite problem at times, where it's pulling extra bobbin thread up to the top at some of the direction changes when I'm using the robotics.
I just got a TOWA gauge and discovered my favorite bobbins won't pull uniformly. They are the five that came with the machine and they are a bit heaver and have a bit of a curve to the bobbin sides. The ones I picked up afterwards are lighter and very flat, and they pull evenly. So I figure a lot of my sporadic tension problems were do to those bobbins.