I have found my screen blank at random. Mouse pointer appears upon wake, but is frozen. I have to do a hard restart to fix. 2011 27" iMac 3.4Ghz core i7 24GB RAM. Any fix to this?

I have randomly found my screen blank upon waking the display several times over the past few weeks. Occasionally, the mouse pointer appears, but is frozen in place. The only way around it is to do a hard restart (pushing and holding the button on back of the computer and shutting it off, then restarting). When I say blank I mean this: I walk up to computer, which has display off...I move mouse to activate the display. The display "lights up" but is blank/dark. A few times I've had the mouse pointer appear, but it is frozen in place.

2011 27 inch iMac

3.4Ghz core i7

2GB Video Ram

24GB system RAM

250GB SSD main drive

1TB HHD secondary drive

OS: Mavericks 10.9.2

Running Parallels with Windows Vista occasionally.

Peripherals:

HP w2007 monitor (secondary display)

Belkin USB Hub

HP Deskjet F4140 Printer/Scanner

Davis Vantage Pro2 weather station console

2GB Apple Time Machine back up drive

USB Keyboard

Bluetooth apple mouse

Bluetooth apple trackpad

Verizon FIOS Actiontec Router

Apple TV (Wireless)

CyberPower UPS