It sounds most likely that your motherboard is grounded by the case. When the motherboard is facing down gravity is probably pulling it away from the case. If it's an old computer that's been in that case a long time then that might not be the case. Check all your spacers, screws and washer(if you're using washers)

Another thing to try is to disconnect the drives and fans one at a time. Do the hard drive last because it might not boot at all without it connected. If you, or a friend, have a spare HD you could toss in there that would work too.

If none of that works check back and I'll keep trying.

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