Signature verification error running apt-get update - Ask Ubuntu

the follow error occurs when running apt-get update:

a error occurred during signature verification. repository not updated , previous index files used. gpg error: http://za.archive.ubuntu.com precise release. failed fetch http://za.archive.ubuntu.com/precise/release 

i running update fresh install. configuration change made change network dhcp static.

here error output apt-get update command:

w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_main_source_sources  hash sum mismatch  w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_restricted_source_sources  hash sum mismatch  w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_universe_source_sources  hash sum mismatch  w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_multiverse_source_sources  hash sum mismatch  w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_main_binary-i386_packages  hash sum mismatch  w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_restricted_binary-i386_packages  hash sum mismatch  w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_universe_binary-i386_packages  hash sum mismatch  w: failed fetch bzip2:/var/lib/apt/lists/partial/za.archive.ubuntu.com_ubuntu_dists_precise_multiverse_binary-i386_packages  hash sum mismatch  e: index files failed download. have been ignored, or old ones used instead. 

here /etc/apt/sources.list file:

#   # deb cdrom:[ubuntu-server 12.04.5 lts _precise pangolin_ - release i386 (20140806.1)]/ precise main restricted  #deb cdrom:[ubuntu-server 12.04.5 lts _precise pangolin_ - release i386 (20140806.1)]/ precise main restricted  # see http://help.ubuntu.com/community/upgradenotes how upgrade # newer versions of distribution. deb http://za.archive.ubuntu.com/ubuntu/ precise main restricted deb-src http://za.archive.ubuntu.com/ubuntu/ precise main restricted  ## major bug fix updates produced after final release of ## distribution. deb http://za.archive.ubuntu.com/ubuntu/ precise-updates main restricted deb-src http://za.archive.ubuntu.com/ubuntu/ precise-updates main restricted  ## n.b. software repository entirely unsupported ubuntu ## team. also, please note software in universe not receive ## review or updates ubuntu security team. deb http://za.archive.ubuntu.com/ubuntu/ precise universe deb-src http://za.archive.ubuntu.com/ubuntu/ precise universe deb http://za.archive.ubuntu.com/ubuntu/ precise-updates universe deb-src http://za.archive.ubuntu.com/ubuntu/ precise-updates universe  ## n.b. software repository entirely unsupported ubuntu  ## team, , may not under free licence. please satisfy  ## rights use software. also, please note software in  ## multiverse not receive review or updates ubuntu ## security team. deb http://za.archive.ubuntu.com/ubuntu/ precise multiverse deb-src http://za.archive.ubuntu.com/ubuntu/ precise multiverse deb http://za.archive.ubuntu.com/ubuntu/ precise-updates multiverse deb-src http://za.archive.ubuntu.com/ubuntu/ precise-updates multiverse  ## n.b. software repository may not have been tested ## extensively contained in main release, although includes ## newer versions of applications may provide useful features. ## also, please note software in backports not receive review ## or updates ubuntu security team. deb http://za.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse deb-src http://za.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse  deb http://security.ubuntu.com/ubuntu precise-security main restricted deb-src http://security.ubuntu.com/ubuntu precise-security main restricted deb http://security.ubuntu.com/ubuntu precise-security universe deb-src http://security.ubuntu.com/ubuntu precise-security universe deb http://security.ubuntu.com/ubuntu precise-security multiverse deb-src http://security.ubuntu.com/ubuntu precise-security multiverse  ## uncomment following 2 lines add software canonical's ## 'partner' repository. ## software not part of ubuntu, offered canonical , ## respective vendors service ubuntu users. # deb http://archive.canonical.com/ubuntu precise partner # deb-src http://archive.canonical.com/ubuntu precise partner  ## uncomment following 2 lines add software ubuntu's ## 'extras' repository. ## software not part of ubuntu, offered third-party ## developers want ship latest software. # deb http://extras.ubuntu.com/ubuntu precise main # deb-src http://extras.ubuntu.com/ubuntu precise main 

try following commands

$ sudo apt-get clean

$ sudo mv /var/lib/apt/lists /tmp

$ sudo mkdir -p /var/lib/apt/lists/partial

$ sudo apt-get clean

$ sudo apt-get update