This is way wierd to me.  I have an HP7955, 16.GHz P4, 512 MB RAM running Win XP Home.  It has an NVIDIA video card, no other expansion cards.
It locks up during startup.  No response to mouse or keyboard and the time doesn not change. But when I set on its side, it works just fine almost every time.
I have:
replaced memory several times, using different DIMM sockets
replaced the hard drive
removed the extra expansion cards - modem, IEEE1394 card
swapped the video card for another similar card and updated the video driver
all connectors are tight

It has on occasion, locked up during boot, during startup, even in safe mode. It sometimes fails to boot at all. HDD LED comes on but nothing starts.
I have removed everything I can from the startup list, leaving only those programs that look like basic Win system programs.

The most consistent item is that it works reliably when on its side, most often locks up when upright.  Nothing is moving that I can see.
My best guess now is that there must be some stress on the motherboard that changes when it's on its side.
I have been struggling with this machine for several weeks now, and can think of nothing else to try. Othe than replacing the whole machine.
Any other ideas?

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