Apologies for the thread necromancy, but after years of trying to solve this problem, I finally found the root cause, and this thread is one of the first ones that pops up if you search for intermittent iDRAC problems with LibreNMS.
iDRAC 6 and up have a rate-limiting firewall rule enabled by default that cannot be configured through the WebUI (except on iDRAC 9, and even then, it’s buried in the menus and disguised as the automatic lockout feature for password retries)
to fix this, SSH into your iDRAC (or use racadm on the host’s OS) and run the following:
racadm set iDRAC.IPBlocking.BlockEnable Disabled
Your SNMP queries will no longer sometimes time out. With this enabled, the iDRAC will throttle SNMP queries to around 3 per second sometimes, but not always. With it disabled, it does not.
It took an absolutely ludicrous amount of effort to get this answer from Dell, and even after months of back and forth they still won’t directly admit that this switch controls more than just the failed password lockout.