Mountain Lion freezes after sleep on iMac (Mid 2007) After upgrading to ML, my iMac freezes when going to sleep. The fan goes off but i can't not wake it up. Its that weird i need to unplug the power, and then i can start it again.

Keeping the power-on button pressed is not working.

It does wake-up from the screensaver, but then the mac isn't asleep (yet).

UPDATE: after i set the "Computer Sleep" to never, and "Display sleep" to 30 mins. I did had the "hdd to sleep" and "wake-up" LAN checked. I still got a black-screen, and complete froze, noway to wake-up the Mac. Unplugging the power for a restart.
Please try resetting PRAM and SMC. Also, you may want to run Disk Utility to perform a validate and repair on your permissions and disk to see if that helps.