[CentOS] Unable to change runlevel at boot time

Hello,

   I am running in problem of changing boot mode.

   My snort script dump packets speedily that I am unable to login as
either GUI or CLI

   I have followed steps

   Boot Menu options -> pressing e -> kernel
/vmlinuz-2.6.32-358.6.1.el6.x86.64 ro root=UUID=ac176b7b-cbbf-->

   then press e->

  <TYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet radeon.modeset=0

  when I try to change 0 to 1,3,5 it reverts to 0.

  What should I do now ??

  Need help !!

  I am trying continuously from 6-7 hours!!

  Thanks !!
-- 
*Cheers,
Mayur*.