Which part is the culprit? - No post

[alexnt]

My system  shuts down properly at night but cannot boot easily in the morning. I must unplug hardware to boot but not a specific hardware each time. This goes for about a week now. I tried ram sticks one by one. I tried another set of ram. I tried a different psu. I unplugged the gpu and used the onboard. Disks in different slots. One disk at a time. Cleared CMOS. Downgraded and then upgraded mobo bios. Sometimes it stays at black screen, after that I shut down an tried again. Then it take much longer to post and there is a message "bios is updating. do not shut down or reboot" and it hangs there. After many efforts it will post. The battery is ok. When it doesn't post, sometimes cpu_led lights on and the other times boot_device_led lights on. If I have it powered on for a while and shut down it posts immediately.

I assume the mobo became faulty but I don't get any BSOD during usage or stress tests.

Any ideas?