A capacitor should be soldered from each motor terminal to the motor case (or ground). A third one goes across the 2 motor terminals. Use a capacitor value of around 0.01uF (this is the most common value mentioned by most of the discussions on interference problems I have seen.)


p.s. - a quick search thru this and other forums for the subject "capacitor" will produce an astounding number of hits. All pretty interesting.