tv out from computer

Try to view movies (played on the computer, RM format) on TV. I Used a tv out video card (ATI radeon 7000). It worked fine for small screens. But when I turned the movie full screen on computer, the TV suddenly becomes black (but the video on the computer monitor runs properly).

The other problem is when I watch TV programs received from the internet using the same computer-to-tv connection, every time it buffers, the TV set becomes black.

any suggestions will be much appreciated.