DDJ-SR random drops of volume

Hi! I have this ongoing issue with my Pioneer DDJ-SR dropping volume. Usually about 2 to 3 hours into my sets, Channel 2 sometimes drops in volume. I can inst. double it to Channel 1 and then it sounds ok, but it's a huge pain when at the peak hour stuff like this happens.

My friend, who has DDJ-SB, has the same issue with his controller, but for him it's usually Channel 1.

I've tried to get the controller to Pioneer service, but they couldn't find any hardware issue, asked me to reinstall computer and software, but that didn't help.

I hope you can help, 

Best wishes, DJ Mimicus.