A rebuilt file server is slow while the switch log fills with MAC flaps

The systems team rebuilt SRV1, a Linux file server with two 1G NICs cabled to ASW5 ports Gi1/0/21 and Gi1/0/22 in VLAN 50. Since then, copies from SRV1 run at a fraction of the old speed and stall now and then. A capture on a client shows many out-of-order segments and retransmissions. ASW5's log is full of MAC flapping messages.