yum and "rpm -U" not working

[Margaret Doll]

We are running 2.6.18-128.1.6.el5xen on our server.

Recently "yum update", "yum localinstall TIV*.rpm --nogpgcheck",  and  
versions of "rpm -U xterm" hang.  yum hangs up on the transition check  
after all the packages have been downloaded.  "rpm -U" tells me which  
dependencies that I need, but hangs if I include the dependencies in  
the list of programs to upgrade.

"rpm --rebuildbd" seems to work.

How do I start debugging this problem?