NIC or Switch

One computer had issue with dropped packets trying to access servers on the LAN. Change NIC. Next day 3 more had same issue. Change NICs. Next day 5 more had same issue. Change 4 NICs. But on the last machine, just changed the NIC MAC and it works OK. Different hardware/OS/NICs on all. Users on different floor switches. Only core switch is common to all. No routing because all are on same VLAN.

This has gone on for 10 days. No reocurrance on the new NICs, just new users every day…

Ideas?