I would start a transfer and then it would start to drop to a crawl. The computers involved are a desktop running Ubuntu 11.10 and a Win7 laptop.
Transfer was done in two ways: SAMBA (staring at around 1MBps) and rsync (staring at around 6MBps). BOTH methods start to drop.
Networks traffic was minimised to determine if that was the cause but it didn't seem to affect it. I also turned off QoS. This is an internal network (obviously) so I haven't a clue as to what would be causing the problem.