X.org modeline for Panasonic TH-50PX80A

Sure, the brochure says your panel runs at 1366x768. But pick a 1366x768 mode in X, and you'll get a two black bars on either side of the image.

Turns out the panel actually is 1384x770. Go figure. Here's a modeline for your media centres:

# 1384x770 @ 60.00 Hz (GTF) hsync: 47.82 kHz; pclk: 86.84 MHz

Modeline "1384x770_60.00" 86.84 1384 1456 1600 1816 770 771 774 797 -HSync +Vsync