Yes, you are right. Roberto. Both of Steven's and your posts are essential to solve this issue.
In fact, I am not a beginner of SYS/BIOS. When I see your post, I modified the clock rate in XGCONF. Then the issue is solved!
Later, I made several other attempt. And I find:
1. Clock rate must be set lower than 100M (It has been stated in your post). 80M is a good choice. 100M or higher will cause unreliable.
2. Disable the pre-fetch and insert a little time delay before hardware initialization such as below(note that the time delay is a must):
3. Make sure that you are using tirtos_tivac_2_00_01_23 or higher version. Pre-fetch will be enable again in the EMAC driver. See function "EMACSnow_NIMUInit" in file EMACSnow.c. Here is the code cut out of it.
Then, your issue should be able to be solved! I think no other arcane setting are required. Have a try!
I am thankful to you all, Steven, Roberto, and others who gave me a hand! Thank you, Steven! Thank you, Roberto!
Jason.