Thanks AJ for your detailed response. I found the voltage to be correct at 5.1.
Just by chance I touched the AC line cord were it goes into the XD3 and the same symptom re-appeared. I then discovered that the XD3 cord could not, perhaps due to poor quality control by the cord maker, sit securely into the XD3.
Fortunately I have the MS40 and, by trying the MS40 cord, saw that it fit securely and eliminated the symptoms. I have since decided to add a strain relief for the main line cord to prevent gravity from disengaging the connection.