MacBook display intermittently breaks

Hello everyone

I have a macbook pro (identifier MacBookPro5,1) which I have been happily using for about 18 months, however recenly the screen has been acting up. It will occasionally display lines one pixel wide, but over the whole screen (it's quite difficult to describe so I've attached a photo). This happens seemingly out of the blue, probably once every few hours of normal use, and the only way I've been able to get it to go away is to close the laptop, then open it again - sometimes it only takes a few goes for this fix to work, sometimes a a lot. Does anyone have any idea about what this might be?

Thanks in advance

James