Until now I've tried out several different drivers made originally for other TNT cards. Most of them didn't work at all with my card and which worked that couldn't display the image on the TV screen. I wanted to reflash the BIOS of the card as well, but unfortunately I found out that I was already useing the latest version. Finally I installed back the latest STB driver, at least it doesn't hang up all the time, and I put up with this situation.
Thanks to everyone who tried to help me!