HP ProBook 6570b won't start

[dynaflux]

I have a ProBook 6570b which has worked fine - until now. This week the screen went dark randomly, sometimes it came back but often not and when I pushed the power button the login screen showed up, I could login in and continue to work. This occured 2-3 times a day for a week.  I tried to do a clean install with a new but the problem persisted. 
And since yesterday it won't start at all - when I press the power button the HP logo shows up (and text about interupting start up process in the left lower corner), after two seconds it shuts down and start again - trapped in a loop. 
I can press Esc - I have tested RAM and the disk, I also made a BIOS factory reset. 

The Caps lock and Scroll blinks one time which should indicate CPU problems. I swapped the CPU with one that's supported and I'm sure it's ok. 

I monitered the CPU temp when it was running but pretty normal since I just ran office programs. 

Any ideas? 

[Jupitero]

Hello.

The screen going dark part - if this happens again, I'd test with an external VGA/Displayport monitor whether they still work. Also, play a video or cause other movement in the display (move a window with a mouse or whatever) and look close at the display. If you can see the movement, the cause of darkness is the LED backlight not working anymore. In that case you would need to replace the panel. Or fixing the electronics - though with that I cannot help you.

The restart loop - I recommend testing with a bootable USB or e.g. Windows Install DVD media.

If they work the, reason for the restart loop is an error in your operating system boot files, boot sector or such.

If your computer still keeps restarting with a valid boot media the reason must be a hardware error.

If your laptop reports CPU faulty (extremely unlikely) even after replacing with another one, the problem is probably in the system board. If your laptop can still function to some degree, try a BIOS update just in case it fixes *something* related to this problem. Paying any serious money for a system board does not sound very economical for a 10+ year old laptop...

[dynaflux]

I didn't ment there's anything wrong with the display/graphics - I first suspected that but must be related to the "main issue" - that's how it started. 

I have tried to boot from an USB with installation - but the disk isn't identified. I was pretty sure the SSD was ok but I swapped it - without success. That's really odd since I could run the built in BIOS disk test.... hmm.... 
So I have tried two CPU's, tried both  RAM sticks alone, different SSD's. 

I'm really stuck here, but I suspect a defective motherboard.