RPMs corrupted

[Per I. Mathisen]

I have a very strange problem.

RPMs become corrupted when downloading them with ftp or http under Fedora 
Core 4 and 5 (different partitions, same box). They are not corrupte when 
downloading with http under Windows XP (also same box). I have tried many 
mirrors, including download.fedora.redhat.com

The RPMs fail the rpm -K check and their md5sum is wrong. Only some RPMs 
fail, however. Some are always good, some are sometimes good, and some 
always fail. I cannot find a clear pattern.

However, the RPMs are fine if I download them with scp, and also seem to 
download fine if I gzip or rename them to something not with the .rpm 
suffix.

The Core 5 partition is a new, clean install. The Core4 partition has
not been updated in quite a while, but used to update fine earlier.

Doing cmp -l on the RPMs that are corrupted against good versions shows 
one of the two bytes 317 377 or 337 377 as differences at various 
positions in the RPMs. Usually each corrupt RPM only has one of these, and 
always at the same position for the same RPM through multiple redownloads. 
In one case I noticed one RPM getting an additional byte pair after a 
redownload but that was gone at the next redownload.

It looks like this:

   [perrin at localhost ~]$ rpm -K procps-3.2.6-3.3.i386.rpm
   procps-3.2.6-3.3.i386.rpm: (SHA1) DSA SHA1 MD5 GPG NOT OK

   [perrin at localhost new]$ cmp -l procps-3.2.6-3.3.i386.rpm
   /mnt/l/rpms/procps-3.2.6-3.3.i386.rpm
     5333 317 377

   [perrin at localhost new]$ cmp -l gedit-2.14.2-1.fc5.1.i386.rpm
   /mnt/l/rpms/gedit-2.14.2-1.fc5.1.i386.rpm
     22089 337 377
     22453 337 377
   2790177 337 377

So far I've this is what I have tried after helpful suggestions from 
people in #fedora on freenode:
  - memtest86 for several hours
  - changing MTU on my network interface
  - rebooting my ADSL router
  - checking ifconfig for errors or collisions
  - rpm -Va and a rootkit detector
  - rpm --import /etc/pki/rpm-gpg/*

None of this fixed the problem or showed anything suspicious.

I tried to unpack the RPMs using rpm2cpio to find what files
were changed, but found no changes in files in the checked RPMs.

Any ideas on what else I can try to figure this out, please? Thanks a lot 
for any help.

   - Per