PIIIDR3 freezes: help!

Now that is one temperamental board! Planned to use it as an upgrade
to SMP from an ASUS P3CLS. At first it would not start up; turned out
it did not like my Kingston RIMMs from the ASUS, so I had to buy
"guaranteed to work in that mobo" set from 4allmemory. Next XP setup
could not find my SCSI160 HD; turned out the BIOS was getting confused
by my IDE ZIP; had to disconnect it for the install, then plug it
back.

Now the system locks unexpectedly, but consistently, usually within
12h from a reset. Current, stripped for freeze-tests, configuration:

CPU0: 800EB/256/133/1.65V S1 CPU1: 800EB/256/133/1.65V S1
10310440-0152 Philippines 90150062-0165 MALAY
i(m)(c) '99 SC458 i(m)(c) '99 SC458

RDRAM: a pair of 256MB PC800 ECC from 4allmemory, interleaved with
CIMMs

BIOS: 1.2 downloaded from supermicro.com
(back when it would not boot with my old memory)

Peripherals: CDRW on IDE0
HD SCSI160 on LVD

Extensions: PCI4 - SB Live! Platinum
AGP - Matrox G400 DualHead

Case/PSU: Antec SX1040BII and 400W w/SmartPower

Jumpers: FSB - 133 (see below)
Audio - disabled
SCSI termination - disabled (see below)
LAN - enabled
overheat alarm - enabled
PS fail alarm - disabled

The system runs fine:

1) with both CPUs, when FSB is set to 100MHz
2) with one CPU in one slot, and a CM in the other, regardless of
which of the two CPUs and which slot, with FSB at 133MHz

It freezes in every other configuration.

Technical support from Supermicro suggested that the dependency on the
FSB speed indicates faulty RAM, but it runs fine at 133MHz with only
one CPU!

Other postings in this NG indicated possible faulty PSUs; people also
swapped video and sound cards (which I did not do) or moved PCI boards
around (which I did not do either), but neither seemed to help (in one
of the two cases I read PSU was at fault, and the in the other it was
the memory).

All of my components (short of the new RAM and the second CPU) were
running perfectly on the ASUS.

I am almost ready to write off the board and get on with my life, and
thought maybe someone here could have an idea of the last thing to
try. Any and all help will be truly appreciated!

--Marcin Struzak