[CentOS] yum screwup

I have several machine running CentOS
I upgraded all of them from 5.3 to 5.4
One machine has a yum problem after upgrading

# yum update
Loaded plugins: fastestmirror
Determining fastest mirrors
  * addons: mirror.trouble-free.net
  * base: mirror.team-cymru.org
  * extras: mirror.team-cymru.org
  * updates: mirror.raystedman.net
addons                                                   |  951 B 
00:00
addons/primary                                           |  201 B 
00:00
Traceback (most recent call last):
   File "/usr/bin/yum", line 29, in ?
     yummain.user_main(sys.argv[1:], exit_code=True)
   File "/usr/share/yum-cli/yummain.py", line 309, in user_main
     errcode = main(args)
   File "/usr/share/yum-cli/yummain.py", line 178, in main
     result, resultmsgs = base.doCommands()
   File "/usr/share/yum-cli/cli.py", line 345, in doCommands
     self._getTs(needTsRemove)
   File "/usr/lib/python2.4/site-packages/yum/depsolve.py", line 101, in 
_getTs
     self._getTsInfo(remove_only)
   File "/usr/lib/python2.4/site-packages/yum/depsolve.py", line 112, in 
_getTsInfo
     pkgSack = self.pkgSack
   File "/usr/lib/python2.4/site-packages/yum/__init__.py", line 661, in 
<lambda>
     pkgSack = property(fget=lambda self: self._getSacks(),
   File "/usr/lib/python2.4/site-packages/yum/__init__.py", line 501, in 
_getSacks
     self.repos.populateSack(which=repos)
   File "/usr/lib/python2.4/site-packages/yum/repos.py", line 260, in 
populateSack
     sack.populate(repo, mdtype, callback, cacheonly)
   File "/usr/lib/python2.4/site-packages/yum/yumRepo.py", line 190, in 
populate
     dobj = repo_cache_function(xml, csum)
   File "/usr/lib/python2.4/site-packages/sqlitecachec.py", line 42, in 
getPrimary
     self.repoid))
TypeError: Parsing primary.xml error: Start tag expected, '<' not found



I have tried 'yum clean all', 'yum --noplugins', 'rm -r /var/cache/yum/*',
deleting all yum packages and reinstalling, and nothing helps
If I copy /var/cache/yum/* from a working machine all is well
I have used md5sum to check the downloaded files and they match the files 
on a working machine
If I disable the addons repo the other repos load ok
Also rpmforge repo breaks yum
What else might I try

Thanks
Richard