Why does my Minn Kota trolling motor slow down after only three hours use?

I start loosing speed ( down to about .3mph) after three hours use. I also loose all directional control at that point. checking the battery shows full to almost full charge. I bought the motor new this spring and it maintained good speed (2.5 mph) for 6 t0 8 hrs. any ideas what the problem might be?|||It sounds as if you are using a car battery not a Marine: it does make a difference. A Marine gives off a steady discharge were a car battery discharges at a high rate then drops to a lower rate even throw it is still fully charged. If that ain't the problem then you have a loose wirer in the motor itself or you need new clamps to hook to the battery: The springs are getting weak.