DJM900 nexus recording to audacity

i just recorded a 2 hour mix - the first hour was perfect but then mass distortion occurred for the second hour - no settings were changed on the mixer or my mac.

anybody know why this happened?

Some application in the background? USB disconnect?

No other applications in the background. If I unplug the usb and connect it again It records perfectly but only for a short while (10-20 min) before the distortion starts again.
Could this be an audacity problem?

Could be... there's still so many unknowns!

Are you using DJ software to mix?

recording directly from djm 900 nexus into audacity

I understand that, but what is the music source? CDJs or a DJ application?

Rekordbox Usb stick via 2cdj2000 nexus

Okay, so not like the computer is running any additional processing that could be causing it. I would recommend you open the DJM-900NXS Setting Utility and pump the buffer up to the highest value; since you're not DJing with the soundcard itself, you don't need low latency. It could be there is something causing the audio interface to run out of buffer space.

Which OS are you running?