I'm guessing the the processor power required to run the HD graphics is affecting the screen display. Although you have replaced the Graphics card, a lot of processing power still goes through the CPU and the information Bus lanes.
Think of it in the terms of a video download, on a HD video, you need 'Buffering top up' from time to time.
Do you have an S-video output on your computer, as that may be a better connection. Most TV's have a S-video input.
Try that, or an alternative is to take the S-video ( or Video + sound phono plugs) from the computer, and connect it to a Video recorder, then out to the TV.
It may not be a perfect solution (standard picture not HD), but better than nothing.