MacBook Pro 15 inch (2011) screen has stripes My 15 inch MacBook Pro recently has horizontal stripes on the screen.
It looks like this:
http://www.youtube.com/watch?v=VaPt6FHJmtg
It happens randomly and sometimes it gets worse (flickering with horizontal lines,stripes)

Does anyone know what might cause this and how to fix it?

No such issue during the past year...it has just started happening since yesterday.
Take it back to an Apple Store or send it in for repair - that sounds like faulty hardware, and you'll still be in warrantee so they'll repair or replace completely free. 

In case they decide to replace it outright (it's fairly new so they may) you'll want to use time machine to snapshot your hard drive for backups.