There is a tension adjustment screw on the back side of the foot control. This adjusts the tension on the cable. Try loosing it up some and see if that helps. My Edge never self adjusted when I took my foot off the peddle. It always held position.

Link below is to the owners manuals. Pick your and download it.
http://www.minnkotamotors.com/Suppor...-Manuals/Edge/
Find the section on adjusting the Steering Cable. On the first manual it is on page 12.