I'm pretty sure the "excessive interrupts" issue was successfully 
tracked down by Jon Burgess (thanks!).  He found this post describing an 
ICH5 hardware issue,
http://www.mail-archive.com/freebsd-stable@freebsd.org/msg58421.html

and he also submitted the attached patch.

I've been meaning to rewrite his patch to isolate it more to ata_piix, 
but in the meantime maybe folks could test this?

	Jeff