shaping like behavior while doing an scp

While scp'ing a large file between two systems on the same network I experienced something similar to shaping, though I was not shaped:

$ scp drupal1* user@host:/var/foo/
drupal1_data.img.gz                             2% 3122MB  33.6MB/s 
...
drupal1_data.img.gz                            20%  294MB   0.0KB/s -
stalled -Write failed: Broken pipe
lost connection
$

The ascii illustration above isn't a direct paste but should convey what happened. I watched 33.6MB/s show down to a complete stop. What's odd is that I was then able to do the scp again and maintain 33.6MB/s and get the scp done. However, I also saw the issue above now and then intermittently. I was going from Fedora 12 to RHEL5.5. I'm curious and want to look into this more. Someone suggested I would find relative information at High Performance SSH/SCP - HPN-SSH

Update Dec 2011::
A solution to the above is posted at linuxsecure.blogspot.com. It comes down to adding:

net.ipv4.tcp_sack = 0

to /etc/sysctl.conf and then enacting it with "sysctl -p".