[CentOS] yum update fails: CentOS 5.2

It gets as far as telling me:

Total download size: 207 M

It doesn't ask if its ok.  But
if I type y<enter> it reports:
   Is this ok [y/N]: Downloading Packages:

Then it does nothing.  I can't kill it
with CRTL-C, but I killed the yum process
with kill -9 from another window.
I retried it several times; same result.

Thanks for your help.

Mike.