Weirdness - FBSD 7, Routing, Packet generator, em taskq

[Paul]

I have a FreeBSD router set up with Full BGP routes and I'm doing some 
tests on  using it for routing.

7.0-RELEASE-p1 FreeBSD 7.0-RELEASE-p1 #6: Thu Apr 17 18:11:49 EDT 2008  
amd64

oddness..:

Use a packet generator to generate random source ips and ports and send 
traffic through the router to a destination on the other side, single ip.
What happens is the 'em0 taskq'  starts to eat cpu... but the funny 
thing is immediately when I start the traffic (say, 100,000 pps) em0 
taskq is about 15% cpu.. and then over the course of 2 minutes or so it 
climbs to 60% cpu..  This makes no sense.. The packets per second are 
continuous and it just routed 100kpps for 60 seconds with less cpu so 
why in the world would it slowly climb like that? 

It's an observation I suppose and I was hoping if someone could 
enlighten me on WHY.. :)   I did test it on 3 different machines by the way.
It even does this with just a handful of routes in the routing table , I 
tried that too just to rule that out.  
I don't remember Freebsd 4/5 doing this??

Thank you.

Paul