Did you ever find a solution, I have same problem.
I believe it is related to EMI/RFI interference probably from your power supply for the laptop (unplug it and you'll see it disappear). I think that elminates the possibility of it being the VGA cable (although you need ones with Ferrite cores -- those black tubular things that are right before the actual connector on both ends.). I just bought a monster cable power strip with Power Clean technology. I tried Belkin and APC who claim to have ~40 db protection from EMI/RFI, but they didnt work. I'm hoping the Monster Cable one will clean the power so it eliminates this electro magnetic problem. Otherwise maybe I need a better VGA cable?