Sounds like a support request may be the way to go. Perhaps disabling topology polling could provide a workaround- go to "list resources" for the node and deselect.
There is a bug in earlier version of Cisco operating system code that causes intermittent no responses via SNMP. It also causes intermittent CPU spike when downloading configs via SNMP. Upgrade to some of the later OS versions and it will be fixed
Thanks for reading,
System version: 5.2(1)N1(1b)
The version containing the SNMP bug is 5.0(3)N2(1)
I think we should be covered for the bug.