Cisco Switching/Routing :: 7609 / Interface Traffic Is Far Less Than Its Capability

The top device of my network is cisco router 7609. There are two part subnet of my network, each part use same device type, same running-configs and same network topple: sw6506(to campus)--->sw3560(to buildings)<--->linksys sr324(to offices). IP addresses for manager vlan is 192.168.1.0/24.Suppose we name two part subnet as A and B. the problem is from 7609 I can telnet to every device of part A quickly, but when telnet to  each sw3560 of part B,it responses very slowly. And only sw3560 of part B are response slowly, other devices of part B are ok.If I telnet to linksys sr324 first, then from linksys sr324 telnet to the current sw3560, it's ok.I try to capture packets of manage vlan, but there seems no strange things in it.No users of part B report problems, it seems the network is running well. Compare two sw6506s, the only diffirent thing is, there are "overrun" count at each interface in use of  part B's sw6506. Each interface traffic is far less than it's capability, but it's "overun" count still increasing at working hours everyday.