DELL E6420 Boot problem

Hi all

Past few weeks i have a problem with booting my latitude. If i turn on notebook usually, screen just blink twice and then start to be on still but nothing happened - i include video of this

But i found a solution how to boot. Due FN+POWER i will put notebook into diagnostic and if i quit diagnostic i could restart and then boot properly. I also do a full diagnostic and nothing, everything looks fine. im including also video of this as a proof.

After i did full diagnostic, i also did an BIOS update (just to be sure) to A21 but no change. 

Did anyone have experiences with this?