If your playback is stopping in X2 m,it will stop in X3, Because, its not a problem with Sonar. Its a problem with your sound card, sound card drivers, sound card configuration and/or your PC.
Playback never stops with me and i have the same program as you do. This means that it something other than Sonar, like your sound card configuration, as this plays a 51% role in how Sonar performs. Your PC is the other 49%.
CJ