The issue could be one in a million items. Start with the basics. Do the 30 workstations have something in common. Are they all on the same switch, floor, vlan....
I'm assuming layer 1 is working because it comes up when you reboot, but I would double check that connections are tight and that cabling is not being pulled or hanging. Then go on to layer 2. It's a new roll out but I would check that the NIC drivers are the most recent. Still having issues move onto other layer 2 components like forcing the NIC and switch to 100/Full Duplex or whatever you choose just be sure they are hard coded to be the same. Spanning Tree PortFast should be enable on the workstation port. But before you start messing with spanning tree learn it. My favorite LAN book with a good spanning tree section is "Cisco LAN Switching". Depending on the answers to the first question you might want to do a sniffer trace, look at the spanning tree design and so on. Keep on that path and methodology of one change - test results, one change - test result.
Good luck
