A campus-wide slowdown that stops whenever the warehouse converters are rebooted

The warehouse switch ASW7 has only copper ports and reaches both distribution switches over old multimode fiber through two pairs of copper-to-fiber media converters. The campus still runs classic PVST+. DSW1 is root for VLAN 10 (priority 24576), DSW2 has 28672, and ASW7 is at the default. Twice this week the whole campus slowed to a crawl for several minutes, and both times it recovered when someone power-cycled the warehouse media converters. It is happening again now.