Nvidia Windows vista DVI to HDMI Problem!!

[goonies]

I have an Nvidia Geforce 7800 GT Graphic card working in Windows Vista. When I am playing back a video with the DVI Connect to my Sony HDTV with HDMI I get a line going across the screen sometimes when there is a fast change of picture. But when I change the Screen resolution on my P.C. Monitor to 800x600 it seems to stop. But when I change it back to 1024x768 the problem is still there! Do I need a graphic card with more Memory? My graphic card has 256 already!