Simple UART Echo Program

I Have been tested the code. Its working fine with small echo data.
But My WMP100 kept restarting when send bigger data (estimated ~ 4KB).
Any body can explain why this happen?

Here is my system info :
-“Developer Studio”,“2.1.1.201201180944-R8656”
-“Open AT Framework package”,“2.37.0.201202280640”
-“Open AT OS Package”,“6.37.0.201202060950”
-“Firmware Package”,“7.47.0.201202010317”

Regards,

Aris