yum update errors

I'm trying to follow these instructions:

http://www.brandonhutchinson.com/Upgrading_Red_Hat_Linux_with_yum.html
to upgrade to FC4.

However, I'm starting to get errors with yum - I've tried removing it 
and reinstalling it, but still get the same errors (see below).

    [root@az ~]# yum upgrade mkinitrd
    Setting up Upgrade Process
    Setting up Repo:  updates-released
    repomd.xml                100% |=========================|  951 B    
    00:00    
    Setting up Repo:  extras
    repomd.xml                100% |=========================| 1.1 kB    
    00:00    
    Setting up Repo:  base
    repomd.xml                100% |=========================| 1.1 kB    
    00:00    
    Reading repository metadata in from local files
    updates-re: ################################################## 930/930
    Traceback (most recent call last):
      File "/usr/bin/yum", line 8, in ?
        yummain.main(sys.argv[1:])
      File "/usr/share/yum-cli/yummain.py", line 68, in main
        result, resultmsgs = base.doCommands()
      File "/usr/share/yum-cli/cli.py", line 390, in doCommands
        return self.updatePkgs()
      File "/usr/share/yum-cli/cli.py", line 766, in updatePkgs
        self.doRepoSetup()
      File "/usr/share/yum-cli/cli.py", line 78, in doRepoSetup
        self.doSackSetup()
      File "/usr/lib/python2.3/site-packages/yum/__init__.py", line 93, in 
    doSackSetup
        self.repos.populateSack()
      File "/usr/lib/python2.3/site-packages/yum/repos.py", line 203, in 
    populateSack
        dobj = repo.cacheHandler.getPrimary(xml, csum)
      File "/usr/lib/python2.3/site-packages/yum/mdcache.py", line 103, in 
    getPrimary
        return self._getGeneric('metadata', location, checksum)
      File "/usr/lib/python2.3/site-packages/yum/mdcache.py", line 90, in 
    _getGeneric
        databank = self._unpickle(piklfile)
      File "/usr/lib/python2.3/site-packages/yum/mdcache.py", line 62, in 
    _unpickle
        obj = cPickle.load(infh)


    EOFError
    
    And even with a normal update:
    
    [root@az ~]# yum update
    Setting up Update Process
    Setting up Repo:  updates-released
    repomd.xml                100% |=========================|  951 B    
    00:00    
    Setting up Repo:  extras
    repomd.xml                100% |=========================| 1.1 kB    
    00:00    
    Setting up Repo:  base
    repomd.xml                100% |=========================| 1.1 kB    
    00:00    
    Reading repository metadata in from local files
    updates-re: ################################################## 930/930
    Traceback (most recent call last):
      File "/usr/bin/yum", line 8, in ?
        yummain.main(sys.argv[1:])
      File "/usr/share/yum-cli/yummain.py", line 68, in main
        result, resultmsgs = base.doCommands()
      File "/usr/share/yum-cli/cli.py", line 381, in doCommands
        return self.updatePkgs()
      File "/usr/share/yum-cli/cli.py", line 766, in updatePkgs
        self.doRepoSetup()
      File "/usr/share/yum-cli/cli.py", line 78, in doRepoSetup
        self.doSackSetup()
      File "/usr/lib/python2.3/site-packages/yum/__init__.py", line 93, in 
    doSackSetup
        self.repos.populateSack()
      File "/usr/lib/python2.3/site-packages/yum/repos.py", line 203, in 
    populateSack
        dobj = repo.cacheHandler.getPrimary(xml, csum)
      File "/usr/lib/python2.3/site-packages/yum/mdcache.py", line 103, in 
    getPrimary
        return self._getGeneric('metadata', location, checksum)
      File "/usr/lib/python2.3/site-packages/yum/mdcache.py", line 90, in 
    _getGeneric
        databank = self._unpickle(piklfile)
      File "/usr/lib/python2.3/site-packages/yum/mdcache.py", line 62, in 
    _unpickle
        obj = cPickle.load(infh)
    EOFError
    
Any ideas?
    
James.