Sometimes it boot up, Sometimes don't

My iMac sometimes boot up normally, sometimes it boot up with a dead display but i can see my teamviewer is running on this iMac and i can hear the chime. The iMac sometimes would freeze after a short while of use, if i force restart it, it will boot with black display(it's like a switch off LCD). It also freeze once, when i try start the mac, it freeze at the apple logo when booting up, the loading bar never came up just stuck with the logo for 8 hours.

When it do boot up normally, i had ran a complete system test, i get nothing from the scan, everything seems to be working. It longer boot up now and dont hear the chime, i only hear the fan spinning. There's also nothing on external display when it do boot up with teamviewer. I done absolutely nothing when it can boot up. Hence i had totally no idea what is wrong with it.

How do i go about troubleshoot this? Would it be the logic board issues?