I opened the case up, and there was a massive amount of dust build up in there.
Got rid of all the dust.

There was quite a bit in the video card fan and on the CPU heat sink.
So that might be the cause of the issue.
Having cleaned the case thoroughly, the flickering lines take about 5 mins after boot up to appear instead of immediately.

So it probably is the video card that has gone.
Or at least is in the process of going.