I could use some help on ideas that might be causing my network 
throughput to intermittently halt for around 20 seconds.

It doesn't seem to happen on short or quick downloads, like loading 
most web pages.  But on some longer downloads after 30 or 40 seconds, 
I will hear a momentary churn of disk activity (sort of a 1/2 second 
buzz), then network traffic drops to zero.  After a few seconds 
(10-15?) it will start moving again. Then the whole cycle will repeat.

Cable Modem  -> NetBSD 1.6.1 on i386, ipf firewall  ->  Mac OS X desktop.

I'm sort of clueless on what I should look at to diagnose this.

Thanks,
Roger