Have posted a few times with some strange problems with my PC convertor
The pc works fine connected to a CRT monitor, or a LCD monitor problem occurs when using the RGB to RGB scart cable that comes with my PC to TV box, what happens is that the picture is jerky, only one res works and that is SD PAL 720 x 576 at 50hz.
This would make you assume that the tv can only handle 50hz but it runs my 360, PS3, Gamecube, Xbox and PS2 games full speed and with no interferance same for DVDS no interferance.
Also the convertor box works for the CRT monitor, no slowdown when connecting from the CRT cable on it to the box its only RGB scart that has the issue.
I would try a regular composite cable but dont have one, I might be able to find a SVHS cable though.
EDIT the issue affects both laptop and desktop so its not a graphics card or PC issue IMO,