I recently got the Toshiba 47HL167. I've noticed a horizontal line at random positions every now and then when viewing videos from my PC. The lines are only noticeable on the TV and not the computer monitor. (I use an HDMI cable from my videocard to the TV.) I have not noticed any lines when using my DVD player though. My video card is the Visiontek HD 2600XT. Is there something wrong with my television or is it my computer?

the horizontal line is most probably the vertical sync. In PC mode, your TV automatically changes to Native Mode (no overscan) so you start seeing these "junk". When watching regular DVD player, your TV is NOT in Native Mode so there is a slight overscan happening and the "junk" got hidden.