Help debugging mysterious freezing (5.4)

[Phil]

I'm trying to debug a strange freezing issue on my file-server, but  
I'm not quite sure how to go about it.

My hardware setup is:
-Rather old P3
-5 HDDs (All Seagate, around 1 year old)
--4 of those drives (all apart from the system drive) are connected  
to a Promise Ultra100 TX2 PCI controller card
-vr network card

My software setup is:
-FreeBSD 5.4
-Custom kernel:
--Workaround from i386/73706 applied
--No APIC
   (Required for my vr network card to work)
--Device hints disabling ACPI and APIC
-The bare essentials running for a NFS/SMB server

What happens, is that the machine will freeze (no HDD activity,  
doesn't respond to keyboard, doesn't respond to network, etc.),  
usually when there's a lot of HDD activity.

I don't get any console error messages, helpful log messages, or  
anything - the machine just halts and I have to give it a hard reboot.

It's been working fine for almost a week (sharing drives over NFS/ 
SMB), when this started happening (I didn't start anything special,  
in fact, I wasn't even using it).

The last couple of times this has happened, has been during a fsck  
(during phase 1) of the drives after I'd noticed it had frozen. The  
next time I ran fsck, it completed fine, no errors.
It doesn't seem to freeze on one particular drive - SMART status for  
the drives is fine, and I've run a few tests - all pass. The IDE  
cables are all 80-pin, nice, stiff, new ones as well.

If this is a hardware problem (which it looks to be), I'd at least  
like to know where.

Thanks,

                     -Phil