Question : Network throughput issues on Windows 2003 server

Here's our basic setup:
Windows 2003 server SP1
various Dell makes
Intel or Broadcom Gb network cards
Dell 5224 Gb switch (version 3.1.0.16)
Cat 5 patch cables (show good quality on Intel NIC diagnostics)

Here's my problem:
On some servers I get great throughput (monitoring using the network tab on Perfromance Monitor), ranging anywhere from 25-50% throughput on the Gb connection.  On other servers I get only 1-3% throughput (again Gb connection).  This copying the exact same file set, using the same procedure (windows explorer) to the same target location.  All NIC drivers are up to date, and all servers have had all the latest patches/updates from Microsoft installed.  They are all connected directly to the same switch, with no more than a 15ft patch cable.  

Any ideas or suggestions?

Answer : Network throughput issues on Windows 2003 server

have you  tried forcing 100.
maybe your cables are not cat6