How to solve mysterious system lockups?

[N. Harrington]

Hello
  I have several systems that are used as squid
caching servers. I have some systems that use SCSI
disks and some  that use SATA disks. They are
identical in everyway except for the sata vs SCSI
drives. 

 At random times, the sata based systems seem to be
freezing. You can ping them and they respond, but you
cannot log in. Nor are any logs processed during that
time. 

 I figure it mist be something to do with the disks,
but I am not sure how to solve it. There seems to be
little rhyme or reason. It does not happen necessarily
during busy times. It can happen in the middle of the
night.

 Any pointers in how to track down the cause would be
much appreciated.

 Tyan S2881 Motherboard - 4gigs mem
 Using 4 SATA (or scsi) drives
 FreeBSD amd64 6.2-STABLE.

 Thanks!

  Nicole