DDJ RB2 Sync "failed" out for some hours

Hello friends of ddj-rb2, yesterday I tested the PIONEER DDJ-RB DJ controller under increased use in the club. After about 3 hours, I was unable to adjust beat power (sync) on one side of DDJRB2, ie. left works  at 120bpm and right at 117bpm. After switching on the "Sync" ... nothing happened. No sync, no tempo change. Then I noticed in the course of the evening that when left (Sync worked all night effective) a song ran at 120bpm, which sometimes "right" automatically adjusted to the right. Strange. Working with MacBook Pro (latest generation including external monitor (21 "), both devices connected via multi-port adapter from Apple.) First I thought of" performance "weakness in the Mac, but the CPU only went on 20% „hot“ whole evening ;-) After a few hours, the left side Sync worked "every now and then" ... Does anyone have have an idea?