no contact with unit

tomus50

i am wondering if anyone else has experienced the same problem.
IRC5 / 6600 stacking robot connected to a Siemens plc via profibus.
equipment  is approx. 7 years old and the error has only recently developed.
periodically getting this error, 41555 : no contact with unit Pbus.
 i have replaced the devicenet pci and the dsqc 352a.
not sure where to look from here.
any suggestions?

thanks