Step down is able to supply max. 3A. But when I measured a current, when servo rotates, ampermeter shows 0,12A. What goes to stepdown is 12 V branch from PC power supply. PC power supply is some no name supply, I dont know what it is, it looks old, it have got more molexes than normal cables 
But PC supply is able to supply more than 3A, so I think that bottleneck is more stepdown than power supply.
Maybe, can I solve that problem with slowly spinning at beginning and then rotate with normal speed?