Intel 82579LM failed loopback testing

I got the board with i5 CPU 3550S and Intel 82579LM onboard NIC .Found intermittent failed running internal & external LAN loop-back test by using Intel LAN DOS Utility Celo v1.17.  The test only failed after running for few cycles , Anyone have idea what cause the test failed?

A latest version V1.21 found in Intel download center, any changes in Celo from V1.17 to 1.21?

Thanks.