Bad Nav or Autosense.  I know the cables have been an issue in the past but per my post below, I had 2 of my 3 nav's go bad last year.  Had the same symptom where it would be pointing in the wrong heading and would never stay online.  Ag express fixed them both.  I believe bench fee is $85 and that with the shipping was cheaper than plugging and playing to try and track down the issue.

Repairs Needed:  

     * Repair Power Supply Circuit to stabilize voltages.  

     * Repair CAN Bus Circuit to improve CAN signal.

     * Repair IMU Circuit to improve gyros performance.

     * Repair Filter Circuit to reduce noise.

     * Install Updates to Main Board

Repairs Needed:

     * Repair CAN Bus Circuit to improve CAN signal.

     * Install Updates to Main Board