Black screen on Mac

Hello,

My Mac (see hardware specification Below ) is running under Sierra 10.12.6.

Since a few days I can work only max an hour before the screen gets black and let me no other option than reboot. Even the mouse is not visible.

My first impression is a problem with the cooling (fans) as the computer seems to be working longer after a long shutdown period. But I cannot be sure.

I use a pgm controlling the fans and make them run at more than 2000 rpm but it does not change anything. Sometimes I get the black screen after only a few minutes.

I used the MacOS utilities to completely reinstall a new Sierra OS. It did not solve the problem.

I tried to start in Diagnostic mode ( Startup holding the 'D' key with an ethernet cable connection) but I get a message " apple.com/support -3403D" and cannot complete a diagnostic.

Can somebody help me on this issue ?

Thank you

Eric

Hardware Overview:

Model Name: iMac

Model Identifier: iMac11,3

Processor Name: Intel Core i3

Processor Speed: 3,2 GHz

Number of Processors: 1

Total Number of Cores: 2

L2 Cache (per Core): 256 KB

L3 Cache: 4 MB

Memory: 12 GB

Processor Interconnect Speed: 5,86 GT/s

Boot ROM Version: IM112.0057.B09

SMC Version (system): 1.59f2

Serial Number (system): W8******DB6

Hardware UUID: ****

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