Severe Latency Issues

Problem: A WindowsNT 4 SP5 BDC (Compaq Proliant 6000 w/ 2 NICS for a total of 8 channels) server has a latency issue on downloads from the server to workstations. It also appears only to be happening with WindowsXP Pro. Example: To download a 1 MB file may take dozens of minutes. Also it takes the copy status bar window around 30 seconds to show. This is on a 100MBs full duplex low traffic network. But at times a file or folder will download properly, though it seems a little slow. We have other NT servers (and Win2K) with no problems.
We have already negated network switch and cable run as the problem by changing out with new equipment.
We have run out of ideas and really need this fixed; any and all suggestions are welcome.