late 2009 iMac won't wake from sleep!

Hey guys, very occasionally about 1/5 times i put my mac to sleep it won't wake up i push the keyboard/mouse the HDD spins up (i can hear it) but i don't get the log-in window instead my screen stays black. I've reset PRAM and reinstalled lion! i honestly don't know whats causing it when i used SL about 6 months ago for about a year i never had a problem! I've seen allot of threads about this on multiple forums. The whole reason i changed from Windows to mac is so i don't get stupid things like this happening! quite frustrating other than this my mac actually works fantastic but having to hold the power button down to do a hard-shutdown every time it goes to sleep to get it to work again is quite frankly a joke.

Any help is appreciated guys!

Anyone?

Let's try a System Management Controller (SMC) reset: Shut down your iMac and remove it from the AC power outlet (pull the plug). Let it remain that way for about 5 minutes. Plug it back in and restart. See if it responds properly to an awake signal.

Also, when waking your iMac you can either tap a key or press the power button on the back. Sometimes pressing the power button works if tapping a key doesn't.

Tried that pal, and pushing the power button does nothing unless i hold it then it shuts down :/

If the SMC reset didn't work, it may be time to take your iMac in to Apple and have them check it out. Before you do though, make sure you installed the latest update from Apple for Lion. (10.7.3) I don't know if it will help, but it won't hurt to install it if you have not done so.