Hi all.
This is the post I've place on the Qlab forum:

A strange phenomenon has occurred in one of my long-running Children's musical. All the music for the show comes from multi-track playback. Each songs with multiple stereo/mono stems are send to an Allen & Heath iLive T112 (8 analogue lines) via a RME Fireface 800 on a Macbook Pro (2.7 Ghz with 8 GB of Ram).

Every morning (yes, it's a kids show – starts at 10am), when my operator turn on the system and runs the overture as a test. The audio will display stuttering and slowing down (it's like a turntable running at a slower speed) and also random loud crackles that sounds like digital noise. Once the system 'runs-in' for about 15 minutes, playback will be fine.

The overture song is actually linked to the opening number (so once overture finishes, immediately follow by opening number). I have 2 mono and 7 stereo stems in the overture running into opening number with 2 mono and 8 stereo stems. I've not experienced this during set up, rehearsals and we're into the 60th show this week (the show runs for 45 minutes), so everyday the problem surfaces immediately upon power up and then resolve after 15 minutes.

What we have done:
The show opened with a lower spec Mac-mini and the stuttering,slowing down and crackling happened during one of the show. Have to stop and reboot computer, problem persists.
The show files has been bundled and running on a dedicated drive separate from the root drive.
When we mix down all the stems into a single stereo output from Q-lab and send via onboard mini-jack output, no issues.

Is this a case of audio buffer being overloaded because of running multiple stems over multiple outputs on the interface? Why does the problem on surface upon powering up and then disappears after 15mins (it's almost like the Macbook pro needs to warm up)?

Thanks for reading and hopefully someone has some insights into this.