In regard to the fading problem when you mix down: try mixing down to a wav file that matches the sample rate you're using in Sound Device preferences. If you're using a sample rate of 48000 Hz, mix down to a wav file with a sample rate of 48000 Hz.
In regard to the crashing problems, wait until it crashes again, then quit Mixcraft and send us your Mixcraft-log.txt file before you run Mixcraft again.
To find the mixcraft-log file, highlight the following line and press Ctrl+C to copy it:
Press the Windows Key+R to pop up the Run box in Windows. Click in the Run box and press Ctrl+V to paste in the line from above. Finally, click OK to go to the location of the mixcraft-log.txt file. Submit an Acoustica Support Request
and attach the log file along with a brief description of the problem.