I decided to delay GPU RMA by few days.
Just to be sure it is not PSU fault i have ordered new one online. Same model RM550X from Corsair.
Should have it on Tuesday. I will test my PC with it so i should know if this is PSU or GPU problem.
It is safest and fastest way to do it.
If new PSU will change nothing i will RMA GPU. If New PSU fix my problem i will RMA old PSU. In 48h i will be know what is wrong. Also it is cheapest way because I can return new PSU up to 14 days after purchase with 100% refund and shop can`t say no for return because of online shopping law in EU
I still believe it is GPU fault but want to be 100% sure it is not PSU