If it sounds like someone is starting and stopping the sound real quick, repeatedly, it could be because the computer has slowed down due to other programs running at the same time.
Beatcraft has to work harder as the layers of sound increase. Try increasing the buffer size or the number of buffers in the Advanced Section of the Preferences Playback tab.
Additionally, you may choose to reduce the # of effects that you are using during playback. Go ahead and mute the effects. You can always turn them back on later. (Effects use significant CPU power.)
Another trick is to reduce the number of instruments. Each instrument has its own separate mix down which costs in terms of your computer"s memory and CPU power. If you don"t need sounds to be separate tracks, try and put them on as few tracks as possible.