I have recently finished my first HTPC (and any PC for that matter...) build. Current setup:
Motherboard: Asus F1A55-M/CSM
CPU: AMD A4
HDD: Adata 60gb SSD
Memory: Mushkin 2x2GB
Case: Antec Fusion with whatever PSU came with it
System boots fine and displays great as long as I connected through VGA or DVI to my monitor. When trying to connect through the tv, it still boots fine. The colours and display are eradic when showing the splash screen and then display drops off completely once the OS has loaded.
Well, it's a bad idea to update the bios, but I can see the frustration.
Do you have any other HDMI output device to test the TV with? Like a DVD player, or another computer?
It could be the TV or the motherboard, as you said, you gotta narrow it down more.
Panasonic or Asus are both suspect.