Other than all the basics of shielding, loose connections/vibration, cable too long (RS232 >100'), the only thing I have experienced is in home-made cables and bad solder joints, If yours is a molded plug DB9 on both ends then you are probably ok on that issue.
Brian.
PS -- Is it happening on the bootup of the panelmate or the PLC, Obviously if you shut down and reboot the PLC you will get LOC.