Some interesting USB console behavior, I can't understand.

hi, All! look:

function lampOnOff(out,q){ 
  if (q==1)out.set();
  else if (q==2){ // flashes for 3 sec's
    out.reset();
    setTimeout(function(){out.set()  ;},500);
    setTimeout(function(){out.reset();},1000);
    setTimeout(function(){out.set();  },1500);
    setTimeout(function(){out.reset();},2000);
    setTimeout(function(){out.set();}  ,2500);
    setTimeout(function(){out.reset();},3000);
  }
  else out.reset();
}
function onInit(){
	setInterval(function(){lampOnOff(B5,2);},4000);
	setInterval(function(){lampOnOff(B6,2);},4000);
	Serial3.setConsole();
}

if I plug USB cable, both B5 and B6 flash simultaneously.
if USB cable unplugged, periods of outputs shift relative to each other.
and start working synchronously again if I put USB back.
I can explain it.. console moved from USB. And I have even no webIDE launched.
How i can make flashing
outputs synchronously without USB?
thanks!
ps: board F4Discovery; Espruino ver 1.67