ModbusIP Slave with Arduino (ethernet)

Hi guys
I am trying to make a Modbus slave with Arduino and having some challenges.
It 'works', but fails after 20-30 transactions with a socket error then timeout. The registers I'm using in the Arduino get reset to zero, but the Arduino itself does not reset

My config is as follows:
Arduino Uno R3
Ethernet Shield with 5100 chip
Ethernet library that ships with latest Arduino IDE
Modbus Library as found here: GitHub - andresarmento/modbus-arduino: A library that allows your Arduino to communicate via Modbus protocol, acting as a slave (master in development). Supports serial (RS-232, RS-485) and IP via Ethernet (Modbus IP).

My app software (sketch) is the example included with the Modbus Library

I have also tried the Mudbus library & examples, with same result. Library found here: Google Code Archive - Long-term storage for Google Code Project Hosting.

I am using a commercialSCADA package, and also ModbusView (a Modbus scanner/troubleshooter/monitor) and get the same result with both, ie socket failure then timeout.
I am polling at 1 second with 5 second timeout.

I am thinking of trying the Ethernet card (based on ENC28J60 chip) that uses a different library ?? I have seen some mention in forums about the 5100 chip & socket issues, but not sure if is my problem?

I have invested a week with trying all the options and not keen to open a whole new set of variants if I can get a simple solution with 5100 shield.

Does anyone have any ideas? or experience?

Thanks
Ross

Do you get the socket error on the PC or on the Arduino? What does the serial console say? Which example do you try with (there are 4)?