15" MacBook Pro freezes
First, back up all the data on your Mac with Time Machine, SuperDuper!, etc.

Second, create an installer of macOS Sierra (latest version) with DiskMaker X or other tools.

Third, reinstall macOS Sierra with the installer, but don't erase the hard disk your Mac. All settings and data will still be there as before.

Just to see if the issue occurs again or not. Try to erase your hard disk and reinstall macOS Sierra, if it happens again. Remember to back up all your files before the operation.