Unexplained reboots with 4.9

[Doug Silver]

I recently brought up an old 700MHz P3 4.9 machine and added a 3ware IDE raid 
card 7506-8 with 4 120Gb drives in a raid 5 array.  It will randomly reboot, 
about once-per-day and even though I'm running a debug kernel, it does not 
leave any crash information (which I assume just means that the kernel did 
not panic and dump core).  After the first few times, I upgraded to a new 
400W power supply.  The machine is not heavily loaded and its primary 
function is for NFS/samba sharing.

Any ideas for trying to figure out what is wrong with this machine?  

Thanks.

-Doug