Also, make sure that your cables - head cables and carriage cables, where ever they are bundled, are lined up. In other words, make sure the edges of the cables are lined up. As this appears to be movement related, I'd suspect the carriage cables. If they aren't aligned, power flowing through a trace in one cable can induce current in another trace that it might be causing this. They're engineered to be aligned and if they are there won't be any induction across the cables.
Good Luck.