Software :: Nagios Reports Host As Down Services?

Since a week or 2 Nagios is constantly marking hosts (servers mainly but also a few Serial-over-IP converters) down for anything up to a few minutes. Typically, all services stay in OK status. Looking closer at a host in such state, it's status information is

CRITICAL - Packet Filtered (<IP address of host in question>)

in soft state. Sometimes services are in critical state with the host in OK status. Almost always the status information is "No route to host". Further checking shows no problems. Rarely this state lasts longer than one check interval. This started after a link was down, putting, correctly, all hosts and services on red for being unreachable. The link problems were solved within a few hours but Nagios only showed this after 2 reboots. Since then the problems has lessened in frequency gradually, from 5 to 10 of the 34 hosts being reported down at any given moment (the same for the 150-ish services monitored) to where I am now, 1 to 5 problem statuses (counting both hosts and services) and the occasional 'all green' screen. A week ago, when the problems were going a week already, Nagios updated from 3.2.0 to 3.2.1. This showed no apparent improvement.

Still, the Host Groups screen is not stable. A read or yellow status initially signified a problem to be looked at, right now it is likely a false alarm to will go away. Whenever the "packet filtered" or "no route to host" is followed up by a ping test, no problems, not even with the slightest delay or packt loss are found.