smp hanging problem

Dear SMP list,

I am running red hat Linux (AS2.1) Kernel:2.4.9-e.40smp
on a Dell Poweredge 2650
Perc/3di RAID 5 (five 72GB disks)
The main applications are
1) NFS exporting home directories to other servers on the second 100mbit 
broadcom interface
2) apache and samba servers using the first broadcom which is attached 
to the Internet.
The system is locking up (hanging) mysteriously (sometimes with kernel 
panic sometime with no message) and it is causing immense frustration.
Over the course of investigating this problem we have :
replaced the motherboard (wrong)
replaced a powersupply (probably wrong)
replaced a dead hard drive(relevant)
and we have spent much time (with Dell's support) updating Linux and all 
software packages and the problem
still persists although it has been stable for a few days now.

If anyone can suggest anything to do I would greatly appreciate it.

Bill