This will be down to a synchronizeation problem with the H + V frequencies.
You are sending a VGA signal to a RGB input and this causes problem. I have a simular problem but the other way around where I am trying to connect a Sky box via RGB Scart to the VGA input on a older generation Fujitsi Plasma using a Scart to RGB cable but my screen will not accept RGB video via its VGA input due to the differance in frequencies used between RGB video and VGA.