XDJ-RX2 distorted sound

 hi, new consolle Rx2, firware updated, Rekordbox updated,link PC/consolle ok.
I start mixing but after 3 tracks the sound comes out distorted (like a "flanger" effect).

the PC has the required characteristics core i7, 4GB ram ecc. ecc.

where is the problem?

Thanks in advance
p.s. with USB pen drive it works regularly