From my experience, I would also think it's a power supply problem, and not a signal
problem. The power leads should be short and heavy. Also, I always put bypass [100 nF]
and electrolytic caps [100-220 uF] across the servo power buss to help filter noise.
All in all, you should probably try different things to get more info. Eg, start with tacking
a pulldown on the signal wire, and see if that helps, on and on.