I have 100+ boxes running FreeBSD 8.4 amd64 RELEASE (p9) with the same configuration.
And only one of them sometimes behaves strangely: load average (generally 4~6, it's ok course box have 8 CPU cores) grows up to 30-40, system running slow and
top starts to print
kvm_open: cannot open /proc/[some_numbers]/mem messages. When load average goes down, such messages not appears anymore.
The question is not how to fight with high la, but what does
kvm_open: cannot open /proc mean? System does not running out of memory, as I see.