It's already third time this computer returns to me with the same issue. It's iMac 27" A1312 with 820-2828 motherboard. Customer uses it for video editing. When he leaves his summer house where this computer is located, after few days computer won't power on. He has to disconnect power cord few times to make iMac turn on. When turned on it works fine. So when he brought it to me for the first time I have replaced power supply and it was working fine. After 2 weeks he brought it back for the same problem. I have replaced power supply again along with display power supply and video card (NVIDIA) and RTC battery. It was working fine for few months, but now came back again with the same issue.. When I connect power cord first diagnostic led lights up. After pressing power button led goes off and nothing happens. After many times of reconnecting power cord iMac will trun on and work fine, but after shutdown it sometimes won't turn on. I have already repalced RTC battery again, reflowed SMC, replaced RAM and HDD (I put SSD), tried another power cord. Sadly no changes...