Uni-Telway between PLC and VFD

I have a Preminum Telemecanique PLC using a TSX SCY 21601 Comunication Card talking to several Telemecanique VFDs using UNI-Telway on a 485 network. What's happening is that the communications work between the PLC and the VFD for a while, (using WRITE_VAR, and READ_VAR functions in the FAST subroutine). Then after a while the comunications just stop working (it seems it can take anywhere from 30 minutes to a couple days for this to happen). The only way I can get them to talk to each other again is to either cycle power to the PLC or hook up with my laptop and stop the PLC then put it back into RUN mode.

I no longer get any updated data from the VFDs and any data I send to the VFDs doesn't seem to get there. The comunication light on the TSX SCY 21601 card keeps on blinking the same way it does when the comunications are working properly. There is no error light on either the PLC or any of the VFDs.

To try to fix it we have only one read or write on at a time, all the registers and and addresses are uniqe to each read or write, the first work of the Maganagement Parameter is set to zero before the read or write is used. And the cable going from the PLC to the VFDs is shielded.

Does anyone know why the Communications would keep dropping out, and more importantly how do I stop them from droping out in the future?