Hi Evan,
At first glance, this seems to be related to PD4-2345650880 - "The snmpMaster process experiences memory depletion of approximately 4096 bytes per SNMP polling."
Notes in the PD indicate that restarting the SNMP process can recover memory, as does a reboot. To restart the snmpMaster process, use this command:
restart process "snmpMaster" You've mentioned that you have EXOS 12.6.2.10 on two of your switches, so I'm mentioning the PD that is for that release. It is fixed in EXOS v12.6.2.10-patch1-2 (and greater)
For 12.5.1.6, you'll need to upgrade to at least EXOS v12.5.4.5-patch1-7 (per PD4-2198529731).
However, all versions of EXOS 12.x are at the end of support. I would recommend that you upgrade to the latest patch version of EXOS 15.3.x, as that is the last release version supporting the X450a/e series. It will also include the aforementioned SNMP fixes.
Again, I'm not positive this is your issue, but it does seem likely.
I hope this helps, and sorry to hear of your troubles.
-Drew