Are you using a switch or a hub to link the machines?
Are you using CAT5 cable throughout? Are they all in good nick?
Does the hub or switch show a "collision" light?
Is the latest machine just using a 10Mb only card?
It could be that the "auto speed negotiation" that 10/100 cards do is not completing properly. Manually config all the cards to 100Mb full duplex.
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SuperMicro P6DBS (dual UW-SCSI) BIOS 2.2, 2*Celery 300a @ 450Mhz, 384MB PC100 RAM
SCSI-A=4.3Gb+9Gb, SCSI-B=Tosh32x CD-ROM, Yamaha4416 CD-RW, Iomega ZIP100, IDE1=4.3Gb
IBM EtherJet 10/100 NIC PCI + Nortel ADSL "modem"
Matrox G400 DH 32Mb AGP + Quantum3D Voodoo2 SLI PCI (CL TNT1 AGP on a shelf)
SoundBlaster Live PCI (not Value)
Win2K build 2195 Retail (not 120-day eval)