What I mean is sometimes loose connection is difficult to locate. It may happen in all devices found inside the case especially when you smack it.
So, it can be in inside the power supply unit, switches, loose sockets, cables and ribbons, clips, and screws. So, you must check these all.
See to it all connections are tight especially the connection from the power supply to the motherboard, hard disk devices, and vga card.
To prevent a short connection between motherboard and the casing. Be sure there are provided with rubber washers or anything that are non-conductive materials.