Hi. I'm having the same problem. I have an IMac circa late 2007. It's freezing up too. So far, it's happened three or four times since I upgraded from Lion to Mountain Lion on July 31st. Thankfully, it hasn't frozen more than that, so I'm crossing my fingers. But I have some important documents to work on and I guess I'm going to have to save every minute or so in case I lose some of it to a crash.
I do have questions though: Does forcing the computer to shut down by holding the power button mess up any of the software or the hard disk in general? Should I run Disk Utility or some other program after every time I force the computer to shut down?
Also, someone said that it's possible, after your computer freezes and you reboot, to have a window display asking to send info of the crash report to Apple. I haven't seen such a window. Is there a way to activate it?
Thanks.