bad downloads during apt-get upgrade

[Steve Kleene <skdeb@syrano.acb.uc.edu>]

On just one of my three Wheezy machines, I often get bad downloads when I run
apt-get upgrade.  This started abruptly seven weeks ago.  The general form of
the apt-get error includes this:

  dpkg: error processing /var/cache/apt/archives/[package].deb (--unpack):
   short read on buffer copy for backend dpkg-deb during [package]

Then I find that dd can't read all of the cached deb file.  In one case,
"ls -l" said the file was 5.8 MB, but

dd </var/cache/apt/archives/[package].deb >/dev/null
  dd: reading `standard input': Input/output error
  ...
  2854912 bytes (2.9 MB) copied, 22.261 s, 128 kB/s

If I remove the bad deb file and call apt-get to reinstall that package,
everything runs to completion.  So I have been able to keep the system
updated.

I thought the unreadable file might indicate one or more bad blocks on the
disk.  A recent boot ran fsck (as it does every 25 boots), and no filesystem
errors were found.

What might be giving rise to these truncated deb-file downloads?  How might I
fix the problem?

Thanks.