ext3 and Software RAID-5: issue ?

[root <root lafleur ts strg sony co jp>]

Hello !

I have been playing around with a new machine, RH7.2, ext3 and software 
RAID-5.

Once in a while, the machine hangs, no message, and the only thing which 
works is the physical reset button.

I have checked whatever I could from FAQs, HOWTOs and did not find the 
problem. How shall I go about it ?

the only thing I have noticed is that 3 out of the 7 disks are on UDMA33, the 
other ones on UDMA 100. Does this have another effect than poor performance?

Here some technical aspects of the machine:
RedHat 7.2 Pro
Kernel 2.4.9-13
RaidTools 0.90-25

RAID array:
mounted as /store, root FS on a separate disk with ext3
7 disks of 75GB, each on its own IDE bus (loads of cabling...)
chunk-size 64k, persistent superblock, no spare disk

Access of /store:
samba-2.2.1a-4
NFS (nfs-utils-0.3.1-13.7.2.1)
web (apache-1.3.22-2)
IMAP (imap-2000c-15)

Besides RAID5, I have another machine running the same setup, and no problem 
ever. 

--Pascal