It sounds like your power supply is tripping out from what you describe. After that happens, the unplugging from the mains is what you need to do to reset them. It could be a faulty power supply, but it could also be damaged cable insulation causing a short circuit, or a faulty component causing an overload. Carefully inspect all the cables. If they're OK, unplug everything except the CPU, RAM and graphics card (with the power disconnected of course). Then reconnect each disconnected item until you find which one is causing the shutdown. If none do then it's the PSU.