You mention that the cable has been checked and is fine, but I've been burned by that statement many times before. I've learned to never underestimate the mischief that subtle cabling problems can cause. I would definately triple-check that the cable is properly terminated, that the length and type is within specs, and that the 485-bus has the correct topology. An excellent resource for rs-485 guidelines is B&B Electronics. They have a section of an app note on rs-485 cable selection that is worth reading. The link is below.
http://www.bb-elec.com/technical_library.asp
Regards,
Greg Caesar
National Instruments,
Applications Engineer