First, thank you for your time and answers. I appreciate your help, and criticism, if there is any, is even better!
The Pixhawk is mounted over the PDB PCB on plastic 1 cm spacers. I tested interference with both compasses also, but something strange happens here. I get inconsistent results. Sometimes it shows huge interference, sometimes less than 10%. In theory, the PDB is designed in a way that it should not interfere with the FC. Nearest current path is 6cm away from the FC. Sometimes testing shows that even at full throttle (160Amps) it gets low interference even with both compasses. The problem is the FC behaves inconsistent with every test (firmware and parameters change). I am trying to find some dependence here.
I arm the drone this way: Drone is on the ground in steady state, I connect the battery, wait for initialization ang GPS lock, then I lift it over my head with my hands and my friend arms it. Then he rises the throttle until is hovers, I losen my grip and let it fly where it wants. Sometimes is hovers, sometimes rotates, sometimes tries to fly away. Haven't caught the reason yet.
The GPS is not vibrating, tested it also.
I will testfly it today and will post the logs here.
Thanks again, and any type of advice is appreciated.