Beagleboard boot but terminal show junk characters.

Hi all,
I began working with beagleboard-xm rev C1 about 2 weeks. At the beginning, everything worked well, The board was booted Angstrom successful, but when I connected BB’s ethernet port with wifi modem to compile, it started have problem. I can’t reboot my BB, minicom just show junk character as my attached file. I don’t know why. I have tried many ways: use other terminal programs ( putty, picocom, gtkterm…), remove ethernet connection, flash new image ( Angstrom, ubuntu) …but none of them works. I guess the reason is serial-to-usb cable, I tried with a new cable, but this time, terminal don’t show anything. I think, BB still boot because the D6, D7 LED beside the sd card keep blinking. What can I do to solve this problem?
P/s: I’m student and this is my final project, so I’m very confused, hope someone can help me out.