If you correct the scratch file issue (I learned about the scratch file about 2 weeks ago here on the forum) and you still have issues, here a a few things I would try:
Check you cable runs. Use category 5 or above type cable / terminations and patch cables. Have a professionalelectrician run and test (with a pair scanner) the cabling runs. In over twenty five years in the hardware business you would be amazed at how many poor performing small and medium sized networks we have straitened out buy cleaning up cabling runs that the 'brother in law - the electrition' did on the side. It should cost you around $ 75 to $150 per drop depending on the work involved. Put a fast disk drive in the 'server' machine (preferably SCSI if multiple users will be accessing it at the same time - a good SCSI card will have cache built into the SCSI controller). The lower the average access time, the better. The higher the RPM 15000 vrs 7200 vrs 5400, the better. 10/100 Switches, not hubs for connection of the systems. At a minimum the 'server' should have a switch port. Use at least a 100 mbps network adapter card (they are full duplex - can send and receive at the same time - 10 mbps cards are half duplex. You can get a cheap switch for under $ 100.00 Hope this helps!
