athlon-mp daily hang 5.2.1

Just about every day, my FreeBSD system hangs.  I set up
dumpdev/dumpdir, but I never get anything in /var/crashes.  I'm not
sure what to do to debug the problem.  It usually happens during a
'gmake -j4...', but has happened at other times as well.  Someone on
freebsd-questions suggested that the CPU(s) might be overheating.
Today I monitored the temperature all day long and it just crashed
during a compile with no change in temperature.

What can I do to get more information about what is causing the hang?

I have the following in rc.conf to try to get a kernel crash dump:

dumpdev="/dev/ad0s1b"
dumpdir="/var/crash"

And I'm running a kernel with debug symbols.  I'm running FreeBSD
5.2.1-RELEASE-p5 with 2 athlon mp processors, using the GENERIC kernel
config file (except I commented out INET6 so that firefox will resolve
domains in a timely fashion): 

ACPI APIC Table: <PTLTD          APIC  >
Timecounter "i8254" frequency 1193182 Hz quality 0
CPU: AMD Athlon(tm) MP 2600+ (2000.08-MHz 686-class CPU)
  Origin = "AuthenticAMD"  Id = 0x6a0  Stepping = 0
  Features=0x383fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,MMX,FXSR,SSE>
  AMD Features=0xc0480000<MP,AMIE,DSP,3DNow!>
real memory  = 1073217536 (1023 MB)
avail memory = 1033003008 (985 MB)
FreeBSD/SMP: Multiprocessor System Detected: 2 CPUs
 cpu0 (BSP): APIC ID:  1
 cpu1 (AP): APIC ID:  0
ioapic0 <Version 1.1> irqs 0-23 on motherboard

Thanks.

Patrick