Mac Mini 2012 - Crash with flickering lines

My Mac Mini 2012 crashes without warning. It seems to happen more frequently with (slightly) increased usage - listening to music, watching videos, multiple Safari windows open etc) - nothing heavy like gaming or graphics driven programs though.

I am currently running Yosemite although it did the same on previous operating systems too.

When it crashes, there are lots of small purple/coloured lines around the screen (as per picture below - don't know why it uploaded upside down!).

Sometimes it remains like this, sometimes it goes black shortly after and sometimes it restarts.

Does anyone have any ideas? Much appreciated in advance!