if the wiring is in good shape the voltage variation shouldn't be that much,.5v at the most. If your getting more than that then you have bad wiring or bad connection somewhere, or worse case a bad carriage board. Also check to see if any of the height limit switches are around the height you specified earlier, maybe one of them is glitching the system causing it to throw the code. I'm still banking on the pot just being out of range but since I'm not there to see it for myself i cant say with any certainty.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.