I have an early 2011 Macbook Pro. I'm running a Dell 2407 monitor using a VGA adapter and I get intermittent wavy lines on the external display. If I open the display menu on the monitor itself while the problem happens, the overlaid menu is not 'waving'. If I adjust the pixel clock on the monitor while the wavy lines are there, this temporarily resolves the problem
I'm using VGA as the DVI connection had other issues - intermittent noise / snow. I will probably replace the Macbook later this year and maybe even upgrade to a new thunderbolt display but in the meantime, this is incredibly annoying.
This video shows the wavy lines I'm seeing (but this was not uploaded by me) - http://www.youtube.com/watch?v=yEyUk6S07iQ
I've plugged the monitor into a completely separate power source and can't identify any other source of interference.
I'm running OSX 10.8.2 Mountain Lion. The display adapter is an official apple one.