Radeon HD 4850 Problem

Hello, I've had my HD 4850 for a couple months now and I've had the same problem for a while. When I connect in through RGB to my Vizio VW37L it looks great... well, for RGB at least. But when I hook it up through HDMI, it looks HORRIBLE! I've tried resolution changes and I've searched the internet many, many times. This is a big issue because it causes the card to heat up quickly and then not work. I'd like to have it look nice and have it hooked up through HDMI. That way I can get the best picture quality possible. If I can give you more details to help me, then just ask.