I'm copying in my answer from another post as the words "Mountain Lion", "Freezes" and "Sleep" instantly raise a few flags for me that suggest its all the same issue:
I'm going to take a wild stab in the dark guess and say that this is related to the highly documented "Mountain Lion wake bug" (at least that's what I've dubbed it.)
It's been discussed in a variety of threads on apple support forums, but probably the most successful I found was this one. I had the problem too and I found that there was one simple fix on this thread.
Basically, the theory (and I'm 99% certain it's correct) is that the bug relates to bonjour, and some apps that use bonjour in mountain lion are causing wifi processes to hang and thus crashing OSX. Anyway, the program that was doing it for me, and probably what I've seen to be the most common one, is Transmit, the awesome little FTP client by Panic. Unfortunately, Panics Menu Bar icon which can be quite useful appeared to be the worst culprit, but toggling off "Show Transmit Disk in menu bar" in transmit preferences solved the problem.
If you're not using Transmit or if you're still having issue, run through that forum and hopefully you'll find one of the other fixes works for you.
PS. Apple knows about the problem and I think they're working on a fix.