Ernie brings up an important issue that isn't often discussed which is the inteferance caused by non optical environmental factors such as EMI and RFI. Ferrite beads( I call them ferrite cores ) are an inexpensive remedy and often work. Try shielded wire but experiment grounding the drains at either or both ends. Try reversing the polarity of the Xantech power supply. If it was not mentioned before, make sure you tell the client that if the sign was not part of the original design that all your additional engineering is billable. Good luck.