screen goes blank during use

Screen blanked out during netflix watching. Sound continued.

Then again while scrolling through pdf graphics in preview and ibooks.

Three times, the third wide and equidistant grey vertical stripes.

Each time I had to hard shut it down ... that is, i pressed the on button.

started up fine each time.

Anyone else? any clues? Is it the graphics card?

OS up to date with latest yosemite

cheers

Ti

OS X Yosemite 10.10.3

27" Mid 2011

Model Name: iMac

Model Identifier: iMac12,2

Processor Name: Intel Core i7

Processor Speed: 3.4 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 8 MB

Memory: 16 GB

AMD Radeon HD 6970M

Type: GPU

Bus: PCIe

PCIe Lane Width: x16

VRAM (Total): 2048 MB

iMac:

Display Type: LCD

Resolution: 2560 x 1440

Pixel Depth: 32-Bit Color (ARGB8888)

Main Display: Yes

Mirror: Off

Online: Yes

Built-In: Yes