What version and build are you using? If you don't already have Mixcraft version 6.1 build 213 (or higher), give it a try. It's on Mixcraft's download page
If the problem happens again with the latest build, quit Mixcraft immediately and send us your mixcraft-log.txt file, which is located in this folder:
Another way to find it is to click on the Windows Start Menu, select All Programs, then Acoustica Mixcraft 6 (or 5) and finally Logs. Submit an Acoustica Support Request
and attach the log file along with a brief description of the problem.
Here's a short video: Mixcraft Mini-Tip: How To Find Your Mixcraft Log File