I know quite a bit about BX-boards, stuff like the BX-chipset requires 3.3V to work, and that on this board the 3.3V rail is used directly without additional circuitry to generate it for the northbridge. And memory, too.
The processors are P3-600E with default voltage 1.75V. Tried overclocking these individually on a Abit ST6-Raid and 1GHz (166MHz FSB) was no sweat without raising voltage. So I could bet it's not the processors' fault.. Now the dual system is prime stable at dual 500MHz (6x83MHz). I just don't like the thought of 40MHz++ PCI bus on my server..
On to the memory.. 4 sticks, total of 640MB, 2x256MB + 2x64MB. Mixed PC100 and PC133. Latencies set at 3-3-3 at the moment, so not too tight. I could try dumping the small 64MB sticks, see if it helps..
My main suspect is the northbridge. It's an early stepping BX from 1998. Also it has a heatsink glued on that's thinner than a regular greenie. And the northbridge is located right below the heatsink of the first CPU. And it can't run @133MHz FSB. BOOTs, but freezes. Tested with PC133 CL2 mem and multiplier unlocked P2.
I know that later made BX-chips overclock better. I have two extremely well overclocking Abit BX133s, one that could do 199MHz with a huge heatsink and fan on the northbridge, though needed 4.15V voltage (!!). The northbridge on that one was made in mid-2000. Actually, it would be kind of interesting to try swapping the northbridge, put better overclocking one there..

No right equipment or courage for it yet, though.