Problem during system BOOT

Hey,
I have a system in my ARM board and when I put the auto-boot jumper the
equip must boot the linux kernel, but, happens:

+---------------------------------------------+                            
| S3C2440A USB Downloader ver R0.03 2004 Jan  |                            
    

+---------------------------------------------+                            
    

USB: IN_ENDPOINT:1 OUT_ENDPOINT:3                                          
    

FORMAT: <ADDR(DATA):4>+<SIZE(n+10):4>+<DATA:n>+<CS:2>
NOTE: Power off/on or press the reset button for 1 sec
      in order to get a valid USB device address.
                                                                           
    

NAND device: Manufacture ID: 0xec, Chip ID: 0xf1 (Samsung K9F1G08U0B)
Found saved vivi parameters.

##### FriendlyARM BIOS 2.0 for 2440 #####
[x] format NAND FLASH for Linux
[v] Download Vivi
[k] Download linux kernel
[y] Download root_yaffs image
[a] Absolute User Application
[n] Download Nboot for WinCE
[l] Download WinCE boot-logo
[w] Download WinCE NK.bin
[d] Download & Run
[z] Download zImage into RAM
[g] Boot linux from RAM
[f] Format the nand flash
[b] Boot the system
[s] Set the boot parameters
[u] Backup NAND Flash to HOST through USB(upload)
[r] Restore NAND Flash from HOST through USB
[q] Goto shell of vivi
[i] Version: 0945-2K
Enter your selection: 


This USB Downloader isn't normal... How I solve this?
Thanks.

More one thing...
the problem apparently arises when going to boot NAND flash...

More one thing... again!
Sometimes boot normally... Sometimes don't boot and the screen shown
previously appears... :S