Modbus randomly failed message

I am using Modbus TCP and recently added some read registers.... i randomly get a modbus failed message and have to restart emc2 to establish a communucation link. how do i determine the cause of the problem? I am connected to a GE 90-30 PLC

What version of EMC are you using. Recently Modbus was updated to improve things.

I am using the installed version that comes with Ubuntu 10.04. I know that classic ladder has a newer version but how do i install it? I did figure out my problem was a bad ethernet cable.