IMac randomly wakes up to black screen

Ok, I suppose the title says it all, but here's more detail.
We have a mid-2010 iMac used as a family computer with 5 user accounts. 95% of the time there are only two users logged in (me and my wife). Over the past couple of months we've had a problem where sometimes waking the iMac up presents us with a black screen.
Now, when I say a black screen I don't mean a black screen as if it's switched off, I mean a screen where the iMac wakes up and the video card produces a 'black' screen. Sometimes you can see (and move) the mouse pointer around, but that's it - everything else is black. Nothing I try 'wakes' the computer up. Pressing keys on the keyboard does nothing, and trying to do a force quit via the Command+Option+ESC keyboard shortcut does nothing. The only way I've ever found I can wake the computer when this happens is to force a shutdown and reboot.
As for my troubleshooting, I've zapped the PRAM and reset the SMC. Neither solved the problem. I've also disconnected all Time Machine drives for a few weeks while troubleshooting (in case that was causing the problem) but to no avail. I've also ensured all software is up-to-date. There is no indication of a hard drive issue (and both Drive Utility and Scannerz report no problems). I've also replaced Keyboard and Mouse batteries, and have checked Energy Saver settings and tried different things (including having the computer set to never sleep). The screen saver is not activated. And I've checked the login items for both accounts and we each only have two items, neither of which could be the issue.
I am now 99% sure that this problem only occurs when multiple users are logged in. It does not seem to happen if I am the only one logged in. I haven't yet tested to see if it happens if only my wife is logged in, although that would be my next test.
We are running Mac OS X 10.9.5 and have 12GB of RAM installed. 40% of the space on the hard drive is still free. Also, I do have a portable USB3 hard drive connected 24/7 as my iTunes and iPhoto library disk (although I did disconnect that for a while and also only have iTunes or iPhoto running when I am using them).
This has been the setup for ages without a problem, so it seems to me that something 'new' is happening here. As far as I can tell, the only software my wife uses on the iMac which I don't is MS Outlook (although I've never had a problem with using that on my MacBook Pro, and she doesn't seem to have any issues with it herself on the iMac).
Any suggestions anyone? This is now happening regularly enough to be a nuisance!

Hi Linc
So, another week of troubleshooting hasn't got me too far, except that now I've confirmed that this can happen with only one user logged in, although it seems to take a lot longer before it does.
Anyway, I did try to follow the steps in those discussions you referred me to, but the instructions re starting my iMac in Safe Mode don't work for me. What I mean by this is that the computer starts to reboot in Safe Mode but after the progress bar finishes, I am always presented with my usual login screen. In other words, I never get a black screen with a cursor, but my menubar does have 'Safe Mode' displayed in red text on it.
I have also tried the Command-S startup combination to force a restart in single user mode and entered in the commands as described in the discussions. This works okay, but doesn't resolve my original 'black screen' problem.
So here I am seeking other suggestions?
Any help is most appreciated!
Joe.

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