Ever do a 'yum update' from the command line, then after downloading the packages and starting the update it begins to throw errors at you? I have found that this usually means there is something wrong with the cache or the rpm database. The way I have learned to fix this error is simple, just remove the cache and then rebuild the rpm database as root, like this...
rm -rf /var/cache/yum
Once those two commands finish, simply run 'yum update' again and things should be working normally.Good luck!