iMac black screen, and start booting after 30 minutes

My iMac does not boot when I click the power button. There’s nothing on the screen, the external monitor and I can’t hear de chime.

The fans run very slow, and the iMac starts getting very hot. After 30 minutes or more, it starts booting normally, and then, after 20 minutes, the screen goes off again.

I’ve tried to clean the fans (for the overheating problem), reset the NVRAM / PRAM, SMC. Also, to boot in Safe Mode, Recovery Mode, Single User Mode… it doesn’t do anything.

I don’t know what can I try..

Thank you for your help!

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Lets see what the onboard diagnostics tell us. If you can reboot your system and hold the D key to enter into it. What was the error message?