make buildworld: Signal 11; Illegal instruction

I'm trying to do a "make buildworld" on my system:

 FreeBSD PECTOPAH.shenton.org 5.1-CURRENT FreeBSD 5.1-CURRENT #2: Tue
 Jul 1 19:48:37 EDT 2003
 chris_at_PECTOPAH.shenton.org:/usr/obj/usr/src/sys/PECTOPAH i386

And it keeps dying at various points early in the build. It's a
different location each time, some times as soon as 12 seconds, some
as long as 100 seconds.   Most of the time it's a Signal 11, e.g.:

  rm -f .depend GPATH GRTAGS GSYMS GTAGS
  ===> games/pom
  *** Signal 11

But sometimes it complains about Illegal instruction:

  ===> rescue/rescue/client
  rm -f dhclient clparse.o dhclient.o dhclient.conf.5.gz dhclient.leases.5.gz dhclient.8.gz dhclient-sc\
  ript.8.gz dhclient.conf.5.cat.gz dhclient.leases.5.cat.gz dhclient.8.cat.gz dhclient-script.8.cat.gz
  Illegal instruction (core dumped)
  *** Error code 132

This smells like a hardware problem to me.  Oddly, this is the first
off-the-shelf box I've bought in years.  A Dell 600sc with CERC RAID
controller, 256MB DELL RAM.  To this, I added 512MB Crucial RAM.

I've seen this before in heavy builds (mozilla, openoffice, x11) but
now it's really buggin' me.  I'm kinda stuck if I can't make world.

Suggestions? If you think it's marginal HW, do you have any
suggestions on how to test and determine the culprit?

Thanks.