Hi,
There are many reasons why this could happen, it depends also on your files and current system status and OS version....
The issues you are reporting could be related to damage or corrupted project files, at this point what I can suggest is to download and install a different version(based on your current OS and license activation date) to re test, start a new project and confirm if the issue persist or not. NCH customers can access the versions data base from here > http://www.oldversiondownload.com/
Do a search(Ctrl + F) for MixPad and look for a date near to your license activation date, download a version different from your current one, install, restart your system and re test.
Best regards,