Random Black screen, Help please

"have you tried resetting PRAM?:

Shut down the computer.

Turn on the computer.

Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.

Hold the keys down until the computer restarts and you hear the startup sound for the second time (I usually let it do this till it chimes 3 times).

Release the keys."

I have the issue with 'Black Screen' on my late 2010 27-inch iMac. Boots up fine, but then randomly goes black. The iMac is still alive as I can still hear things playing. Someone suggested I try putting the iMac to sleep using the cmd/alt/eject key commends. Well I tried this, and after doing the key commends then touching any key afterwards the iMac came back. Screen came back on. It did however go off again after a couple of minutes, but used the same tactics again and it worked again. Still a pain, but a solution.

"Hi Gerald, my advice is try replacing the keyboard.

I had this problem and have spent weeks trying to fix it, using every suggested fix on every forum I could find. I did endless PRAM and SMC resets, installed smc fancontrol, uninstalled and reinstalled Yosemite. Every fix failed. In fact it just got worse.

My 2008 imac had become virtually unusable. As soon as it powered up the screen went black, and it sometimes got stuck in a loop of trying to restart and would only stop if I unplugged the machine from the wall.

Then I read that someone had fixed this by replacing the keyboard. I had nothing left to try, so tried it.

It worked. Everything is back to normal. No screen glitches at all. It's like a miracle cure.

I hope it works for you too :)"