This is usually caused by 'buffer underruns'. That is, the audio stream to the soundcard is interrupted when a temporary memory-buffer holding the audio data runs dry. This is caused by CPU overloading. Crackles and pops can also be caused by plugins behaving badly due to incorrect settings or design flaws.
To learn more about resolving either or both these issues see the manual sections on...
1. Buffer underruns
2. Plugins behaving badly
Image-Line Support Team