Maximum PC

I have a question. My main rig and my server are connected via gigabyte ethernet, hard wired. It runs through 2 switches, and there are a total of 6 rigs on the network. When I attempt to copy a large number of large files between the main rig and the server it will start the transfer at 50MB/s or so, then after 40 minutes or so it will eventually slow down to 3-4 MB/s. IF I cancel the remaining copys and give it some time, then start copying the remaining files it will go back up to 50MB/s or so. Anyone have any idea why it would slow way down like that?

The main rig runs XP Home
Server runs WHS.

If any other information is needed just let me know...

What is the make and model of the routers and switchs? How hot are they when the slow down happens? Are the 6 rigs all using the network while this is going on? Leaning towards just over working the soho units. How large is the total amount you are trying to transfer?

One is a 8 port DGS-2208 by D-Link, the other is a 5 port SD2005 by Linksys. Both run moderately warm but certainly not too warm to touch. When this happens no other rig is accessing the network, except for maybe sending or receiving a folding WU. It doesn't seem to make much difference what time of day it is, I have tried to run the transfers all day and all night, and the same thing ultimately happens. I don't know how far it gets before it slows down because I don't have the patience to watch it for hours... And the last big transfer was 65 files, each 4.7GB.