Description:
Using PulseAudio (with a null sink and loopback) as the audio input for ffmpeg causes severe distortion in the output after 20-40 seconds of encoding.

Additional info:
Linux 3.4.8-1-ARCH #1 SMP PREEMPT
pulseaudio 2.1-1
alsa-lib 1.0.25-1
alsa-utils 1.0.25-3
ffmpeg 1:0.11.1-1 and N-43565-gaec9390
pavucontrol 1.0-1

Forum thread here: https://bbs.archlinux.org/viewtopic.php?id=147060
Experienced by at least one other user of PA: http://www.linuxquestions.org/questions/linux-software-2/pulse-audio-module-loopback-distortion-942066/

* config and/or log files etc.
Attached.

Steps to reproduce:
Set up a null sink and loopback monitor of sound card output to the sink. Run ffmpeg and use pavucontrol to direct the monitor of the null sink to ffmpeg.
Open the output file or view the stream (if streaming); around 30 seconds into the video, sound will suddenly contain loud glitches, distortions and increases/decreases in speed and pitch.

Workaround:
Modifying /etc/pulse/default.pa to re-enable interrupt-based scheduling (load-module module-udev-detect tsched=0) seems to alleviate the distortion problem but also increases CPU consumption and makes sound unusuable in Wine.

Variables which have been tinkered with to no avail:
(PA) real-time priority
(PA) high priority
(PA/ffmpeg) sampling rate
(pavucontrol) muting different inputs, outputs, removing microphone/USB headset
(ffmpeg) building from git
(ffmpeg) changing audio encoding, trying every thread value from 0-6 (6-core CPU)