[APT] how to debug "Hash Sum mismatch"?

[Tom Roche <Tom_Roche@pobox.com>]

I'm trying to update debian packages for R, for which the recommended
mirrors are listed @

http://cran.r-project.org/mirrors.html

My sources.list formerly contained

deb http://lib.stat.cmu.edu/R/CRAN/bin/linux/debian squeeze-cran/

with which I was able to keep up-to-date for many months. But tonight I'm getting

> E: Failed to fetch http://lib.stat.cmu.edu/R/CRAN/bin/linux/debian/squeeze-cran/r-base-core_2.15.2-1~squeezecran.0_amd64.deb: Hash Sum mismatch

So I changed my sources.list

- deb http://lib.stat.cmu.edu/R/CRAN/bin/linux/debian squeeze-cran/
+ deb http://watson.nci.nih.gov/cran_mirror/bin/linux/debian squeeze-cran/

did `sudo aptitude update`, and got

> W: Failed to fetch bzip2:/var/lib/apt/lists/partial/watson.nci.nih.gov_cran%5fmirror_bin_linux_debian_squeeze-cran_Packages: Hash Sum mismatch

So I changed my sources.list

- deb http://lib.stat.cmu.edu/R/CRAN/bin/linux/debian squeeze-cran/
- deb http://watson.nci.nih.gov/cran_mirror/bin/linux/debian squeeze-cran/
+ deb http://mirrors.nics.utk.edu/cran/bin/linux/debian squeeze-cran/

did `sudo aptitude update`, and got

> W: Failed to fetch bzip2:/var/lib/apt/lists/partial/mirrors.nics.utk.edu_cran_bin_linux_debian_squeeze-cran_Packages: Hash Sum mismatch

Given that CRAN is a fairly reliable organization, and I have just
failed with 3 recommended mirrors, I'm wondering if something is wrong
on my end. How to debug?

TIA, Tom Roche <Tom_Roche@pobox.com>