PC Reboots Standing Up/works on side

My son built a PC and when it is standing up straight it keeps rebooting. When we lay it on its side it works fine. We replaced the power supply but that did not change anything.

 

I wonder if you've got a flaky connection somewhere, or something that didn't get properly seated in its socket. You might try pressing all cables, cards, and chips into their sockets.

While you're in there, keep an eye out for metal chips, naked wires, and other things that have drifted into places that cause electrical problems.

 

Possible short.
Improperly connected hardware or cableing.

Last month I repaired a pc for friend whose son had built, It too would not boot standing up, but laying down it did.
The problem with this one was the MOBO stand offs were not in the correct position, it had one that was contacting the back side of the mobo shorting it out.