Sometimes, when I'm on the internet, my computer goes black then comes back on with pink, green and blue lines travelling vertically across the screen.
Anyone know what's wrong and how I stop it?
Simple answer: check your monitor cables. Beyond that, I'm not sure. Could be a fault in the monitor, does it only happen when you're browsing the net?
Yeah, from the sound of it, the power cord on your monitor is loose. That could happen when your monitor loses contact with it's power cord, and then gets it back. If that is not the problem, then I don't know.