Sounds like you're pushing your machine too hard. Try freezing tracks that you're not actively changing, using the Freeze Tracks option. This will take a lot of the load off of your CPU, allowing you to play back a lot more tracks. You can always unfreeze them to get back where you were.
Make sure you get the latest version of Mixcraft 4.5, there were some issues with freezing tracks in earlier builds that may have thrown your project out of whack.
When it comes to working with audio, the more horsepower your computer has - memory, CPUs, raw speed - the better.