rekordbox dj 4.2.0b very laggy and Crashed twice in 1 hour using DDj-SX

Rekordbox 4.2.0b crashed twice in 1 hour.

first time it happened I Shutdown MBP reset PRAM and SMC

work very smoothly for 45 mins then the wave forms in software and controller started to lag, sound stuttered and finally crashed with error box on screen

Pioneer DDJ-SX

OS X El Capitan 10.11.5 

Macbook pro 2009 2.53 core 2 duo 8gb ram

WIFI & bluetooth off, ONLY Rekordbox app Active.