Ming -
Based on your graphs, is there something odd with the MOSI line. It tries to pull up between data and then is very slow at the end of the transmission. This could cause invalid data to be transmitted. Don't know if this is just a scope problem or circuit issue.
A couple comments on your register settings/commands:
Config2 - you are setting to the default (you don't need to reset to default)
Config3 - you turn OFF the internal reference, so hopefully you are providing an external reference?
Config 5 - you have an invalid setting; PGA gain cannot be set to 000. Use 001 for Gain =1
Additional comments:
Based on use of START command, you are not doing hardware control, so the pin is tied LOW correct?
Based on your sequence, switch the START and RDATAC commands. This is the standard sequence.