iMac screen issues

Hi,

Over the past week I have been having on and off problems with my iMac screen.

It is a mid 2010, 21.5 inch imac and the screen starts to fragment and then it all freezes. I have to force shutdown and then when I start up it appears like the screen shot followed by a white screen. If I do a safe start up, it will eventually begin but then I get the vertical stripes like my second image and then it fragments into a checkerboard effect and freezes again.

I have reinstalled Yosemite and backed up everything but nothing seems to help. Has anybody seen this before?