You say you have UPS on the workstations, but you don't meniton the file server. Even if you are using a peer-to-peer network, you should have a dedicated PC for the file server. Ed's comment on the network hardware needs to be emphasized. Have you tested the network connections, hubs, NIC, cables, shielding? Are you seeing any network errors or excessive retries? As in most "systems", it's only as strong as it's weakest part.
Rick