820-3298 iMac 27" Late 2012 max resolution 1280x720

[No-Clean]

When the iMac starts up and before the Apple logo the screen looks like an old TV searching for channels (Sorry I dont have a better way to explain this)

- Once the logo comes up the logo looks very large.
- The problem happens more often than not.
- There is external video and with that I have higher resolution options.

Already tried known good screen, different LCD cable, clean BIOS and different OS.

I have also seen this in a 27" 2013 model

 

[2informaticos]

Heat GPU until 180-200 degrees, no much.
15-20 sec should be good enough for test.