Re: Coaxial versus Toslink/Optical
I still wouldn't necessarily suspect the toslink even if it were longer. When they convert the digital signal to light, timing is taken into account and controlled. It could just as likely be poor quality modems (converters) as well, as noted by CharlyD.
Also, a speck of dust on the end of the toslink cable or in the toslink jack can lock or partially block the light, playing havoc.
I'm not advocating one over the other, just pointing out some facts, from experience working with fiber.