Yosemite hangs exactly every 60 minutes
Here come in depth solution that fixed same problem that I had, and maybe it will help you and others. First off all there is no magic wand solution for this, but I spend 3 days to fix it, and now it working 2 days with no freezing..

I have:
Mac (27-inch, Mid 2011), OS X Yosemite Public (installed from Maverick)...

Every 1-2 hours my Mac freezes, sometimes in PS, DropBox, Chrome, Creative Cloud... etc.. and I need to hard reset every time... also if I start doing "normal" reset or shutdown (if my mac is not in freezing mode), sometimes it only hangs with black screen and mouse working...

I tried:
- cleaning PRAM
- fix permission
- delete applications that sometimes craches
- remove logitech keyboard &amp; mouse
- work only with safari, not chrome
- and nothing happens

Long story short...
- Boot you Mac in Safe mode, first.. it will take some time, and than start normal Yosmeite
- Then reset or shutdown, it needs to turn off really fast (30 sec max)...
- Than boot again only with Disc Utility (Command+R to boot into Recovery, or hold down OPTION), you cannot do this from Yosemite...
- Than Verify for errors, and fix those errors (approx. 2-3 hours of waiting)
- Again fix permisions..
- Yosemeith is working now, and don't freeze anymore...

Bottom line: Mac is really problematic with file handling, and if you had Maverick with lot's of install/uninstall applications, than Yosemite is even worse, so Fix disk errors, and everything will be ok...

Hope you will fix it this way..