Linux hangs on MPC870 board

Hi,
I ported uboot-1.1.2 on my board , it works fine so far, i can download
kernel image and no problem with mtest
but when i enter "bootm" , after "Uncompressing kernel...OK", no more message appears, but i tested the same image with a reference board(Adder87x), it works fine.

The differences of the two boards is just the flash size(8M .vs. 32M) and
memory size(8M .vs. 64M) and CPU( MPC875 vs MPC870).

I have no idea what is wrong ? SW or HW ?
Can anyone give me some ideas or hints
Thanks in advance!

Best Regards,
Jianwei