Macbook Pro mid 2012 starts only when cooled

Hi!

I see a very strange behaviour of my MBP (i7 2.3, 4gb, non-retina, mid 2012).

After several days of experements I can say that laptop boots correctly only when it is cooled down to the room temperature (20-23c). If it is warmer or the room temp is more, laptop boots not detecting internal display. OS boots and if I connect the externall display it works fine, but no info about internal one present. If I disconnect the ext. display, internal one goes blue and nothing more. I use FinalCut Pro which heavy loads the graphics if manage to boot the computer first time it works correctly under heavy loads an switching the graphics correctly when I surf internet for example. So I can't restart the laptop, it will boot with the black screen, but I close the lid and it goes to sleep mode and wakes up correctly.

I've done SMC and PRAM reset many times, no effect

Perfomed Apple Service Diagnostics tests and laptop passed all the test

Please, HELP!