iMac 21.5" Mid 2010 weird video issue

[No-Clean]

The iMac starts up every time, chimes, if the alt key is held the startup manager shows up on screen, sometimes it starts all the way to the user session, but suddenly the screen goes black, no image nothing and the 4th diagnostics LED turns off when this happens, with external monitor I get image but when the 4th LED turns off the external monitor also goes black, its important to know that when this happens the iMac is still on, but there is no image, sometimes it restarts a few times after the screen goes blank.

At this point I leaning towards the video card or the LED inverter board.

Any pointers will be greatly appreciated.

 

[2informaticos]

If external monitor goes black too, looks like GPU issue.
You can heat it for test, but solution is change it; if you can get a real new chip.
May be a easy way to find another graphics card...