DDJ SX2 Master Volume go up and down every moment without touch the master

hey all , really i get anonying from this prob , the volume in my sx2 go down and Up i mean the master input and i can see the signal in middle about master volume go down and Up , and all my gain of my channel is fixed good and signal of every channel is good is mean the sound good in every channel and also in headphones , when i listen the master channel in headphones i have the same prob of volume go up and down , i have the DDJ SX2 like 3 years