Firstly, here are the main system specs:
2x Pentium III 800MHz
MSI 694D Pro
4x 256MB PC133 SDRAM
420WATT Vantec PSU
In a nut shell, the systems reboots for no apparent reason while starting/installing up any OS when both processors are installed.
I've ran memtest86 for several hours, no errors. I've tested with one stick of ram at a time. I have tried an agp video card and pci. I have tested with no pci cards. I even bought a new powersupply (I was so certain that was the problem).
I've done this with several different operating systems:
Windows 2000 - Gets into the first stage of setup (the blue screen)... The system has to reboot (normal) and then right before the graphical loading screen comes up (black screen with white progress bar) it reboots. At this point I noticed the two processors have different VCore values written on the CPU label, one is 1.75 the other 1.7. I adjusted this in the Bios and it booted into the graphical installer. I turned it off and came back about an hour later to start installing Gentoo (at this point I was thrilled, thought I solved the problem). Back to the same problem 
Windows 2003 - Finishes the first stage of setup and actually gets into the graphical setup part. The systems freezes and stops responding to input in about 30 seconds.
Gentoo - Boots into the setup *sometimes* Sometimes it will crash as the system is running through the rc.d scripts, other times it's fine (for now). I'll be able to partition and format, browse through directories and such but once I decompress one of the tar.gz stage files (fairly large) the system reboots.
Slackware 9.1 - Reboots itself as the installer loads up.
I read my motherboard manual and it turns out his has an onboard terminator... I popped out the second processor and all is well. No reboots and everything works. Put the processor back and the problem resumes. I even switched the processors, both work - when alone.
Right now I have Slackware running. I compiled a kernel with SMP enabled and tried booting with both processors. It loads up a bit (I see text flying down the screen) and then it reboots.
I read about similar problems where the capacitors around the CPU2 socket had blown and replacing them fixed the problem. Unfortunately, mine are not blown (and yes, I mean unfortunately).
I bought this on eBay. The seller has 100% positive feedback so I doubt he sent me a lemon. He had a picture of the system running XP with both processors. It *could* be a chop or old screenshot but I'll include it anyhow.
http://meeps.no-ip.org:3000/multimedia/winmsd.jpg
Here's the /proc/cpuinfo from one of the times Gentoo partially worked. I'm not sure if this is helpful.
http://meeps.no-ip.org:3000/cpuinfo.txt
Yep, different steppings. Intel's documents say there is no problem and this guy I bought it from apparently had no problems.
That's pretty much it. Anyone have an ideas? First one to solve it gets my first born, any takers? 