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.
Best Answer
VGA cables are generally not designed for the high bandwidth required of large, digital displays, so it wouldn't surprise me if this is a cable issue that would be present in a variety of cables. I had similar issues with a KVM adapter. (If you're using a KVM adapter, then the KVM itself is probably the problem.)
One thing to try would be to lower the refresh rate on the monitor to the lowest setting the Mac will allow and see if that solves the problem.
Better would be to get the DVI cable working, but again the intermittent noise/snow points to a bandwidth limitation of the cable (or KVM adapter).