MacBook Pro Freezing
Suggestions:


Run Mac in recovery mode (keep holding ⌘ cmd + R on startup), then try to Repair Disk and Repair Disk Permissions using Disk Utility.
As suggested in Mountain Lion Screen Freeze thread, try OnyX software which allows and to verify the startup disk and the structure of its system files, etc.
Run dmesg and check /var/log/system.log for more detailed errors.
Run Console app to check crash logs including backtraces if any.
Run newproc.d or iofile.d dtrace scripts to find anything suspicious (e.g. sudo newproc.d)


Related:
Apple.SE: Mountain Lion Screen Freeze
Apple Discussions: Mountain Lion Screen Freeze (over 177 replies)