iMac27 crashes to black screen, fans running, won't boot for a day
i've got an late 2009 27'' iMac (Core 2 Duo).
Recently it started to crash to a black screen, after that the fans are still running (like normal, not loud etc.) in the back, but it's not just the screen that crashes or turns off, there is no sound and the Mac disappears from the network. I have to do a hard shutdown to power the Mac off.
When i try to power it back on right away, it will not boot (black screen right from the start or before the OS is booted). I can unplug the Mac and plug it back on, but it won't boot anyway.
When i wait for about 24h or so (completely unplugged) it will boot up just fine and work correctly for days / weeks ... until the next crash.
I tried reinstalling the OS several times, it even crashes while installing the new OS, i tried NVRAM / PRAM / SMC etc. and of course also fixing the file permissions and checking the disk. Everything seems alright from the software-side.
Does anyone have an idea or have you seen this behavior before?
Could it be the Power Supply being defective or could it even be the Logic Board?
Thanks in advance for your help, it's really appreciated!