To prevent HD content piracy the VGA and DVI connection is limited to about 800x600. You can call it in but I've seen this too many times and share my findings.
If yours did allow it be sure you limit the refresh rate to 60 Hz as you try the higher resolutions.
I have samsung 32" 81rb lcd tv and radeon x1950pro vga card. Operating system is Xp pro Sp2 and vga driver is catalyst 7.7. The maximum resolution supported by samsung is 1366x768. I have connected the lcd tv with vga cable and a dvi converter. There is no problem with bios screen and win xp logo screen. There is also signal in xp with resolution 800x600. But when the resolution is increased above 800x600, the signal is lost anda tv is in standby mode. How can I solve the problem? Thanks for your replies.