output slowing down and crackeling

[noSpoon]

While running ableton with seven tracks in clip view playing at the same time the tempo of the song began to slow down and the audio quality began degrading. The assigned tempo did not change the entire time. My cpu usage was only around 50%. Ive run this same set many times so I know the computer can handle it. Once playback was stopped and restarted the problem no longer occurred and was not reproducible. any ideas?