mac book air screen flickering

Hi

Please help, my mac book air, mid 2012, 2 ghz intel core i7, with intel HD graphic 4000 1536mb has recently started flickering.

The screen would flash black, the apple logo on the back of the screen will go off, and the screen will eventually go all black.. almost like starting in the corner then covering the whole screen.

It makes no difference if I'm on battery or ac.. or if the computer is running lots of programmes or just the finder screen...

I've tried running diagnostics, reset the p-ram, and various other reboot options.. nothing has helped...

Any ideas????