Does the "show snmp" counters indicate the problem? Someone on this list previously mentioned a bug with snmp in bigiron software:- "The 7.8.00d is released now the bug is #33598 .The description of the bug is different, but this issue " SNMP uses a different interface IP while responding to a get request " has been addressed in this issue. I verified the issue has been fixed in 7800d code."
To test this you could sniff on your management host for snmp responses coming back from different IP addresses. Otherwise you could drop the interface options from the "snmp-server enable" and lock snmp down with an acl, ie access-list 8 permit x.x.x.x access-list 8 deny any log snmp-server community ....... ro 8
