[CentOS] Yum problem on Centos 5.3 (64-bit)

Hello,

I'm having a yum problem updating a system on Centos 5.3, 64-bit ... 
i.e. 'yum update' returns "No Packages Marked for Update". Problem 
appears to be related to connecting to the mirrors where the 
repositories are located but I could be wrong in that. What's confusing 
me is that I have another system (Centos 5.5, 32-bit) that's working fine.

Best way to illustrate this is to show the output from the ("bad") 5.3 
system when I run 'yum -v list available'

[root at servername yum]#  yum -v list available
Loading "fastestmirror" plugin
Loaded plugins: fastestmirror
Config time: 0.031
Running "init" handler for "fastestmirror" plugin
Yum Version: 3.2.19
COMMAND: yum -v list available
Installroot: /
Ext Commands:

   available
Setting up Package Sacks
Running "postreposetup" handler for "fastestmirror" plugin
Loading mirror speeds from cached hostfile
pkgsack time: 0.000
Reading Local RPMDB
rpmdb time: 0.000
Setting up Package Sacks
pkgsack time: 0.000

Similarly the the first few lines of the output from the ("good") 5.5 
system when I run 'yum -v list available'

[root at other_server_name yum]# yum -v list available | more
Loading "fastestmirror" plugin
Config time: 0.032
Yum Version: 3.2.22
Setting up Package Sacks
Loading mirror speeds from cached hostfile
 * addons: anorien.csc.warwick.ac.uk
 * base: ftp.heanet.ie
 * extras: anorien.csc.warwick.ac.uk
 * updates: anorien.csc.warwick.ac.uk
pkgsack time: 0.029
rpmdb time: 0.000
Available Packages
Cluster_Administration-as-IN.noarch     5.2-1.el5.centos                 
base
Cluster_Administration-bn-IN.noarch     5.2-1.el5.centos                 
base

The contents of /var/cache/yum seems to be the same for both systems, as 
does the file /etc/yum.conf.

Would appreciate any thoughts on this.

Thanks,
John.