Poor network/forwarding performance

[Andriy Korud]

Hi.
Some time ago I was posting here message concerning poor ipnat performance.
However I've found that even without nat (clear forwarding) performance is also
poor - at about 6Mbit system gradually becomes completely unresponsible but
return to normal state immediately after disconnecting network cable.
System is: Xeon 2.8GHz/4-STABLE/Intel gigabit network card (em0). Device polling
is enabled. Mbufs - 1% in use.

And what I've noticed - in that (or near) state top shows 100% usage by top
itself (and sometimes I've noticed 137% CPU usage!?)

Any ideas, where can I look and check?

Best regards,
Andriy Korud