iMac freezes complete, except mouse pointer. What can I do?
I had this problem as well, it turns out that migrating from a prior version was the problem, specifically kernel extensions that used to worked now "sorta" worked until they didn't.

Using Lingon (see the Apple Store), it was possible turn on and off various services to locate the problematic one(s).  Using Onyx it was possible to clear out the kernel caches.

For me it was some historic SnapZPro extensions that needed updating.