Try putting some ferrite beads on the wire coming out of the digital pin, as close as possible to the pin. Beads like these will cut RF interference in the short wave and FM bands: http://www.ebay.com/itm/50-pcs-2643250302-FAIR-RITE-43-MATERIAL-SHIELDED-FERRITE-BEAD-25-300MHz-/121152377726?pt=LH_DefaultDomain_0&hash=item1c353e937e