I have been paying more attention and the dither happens from the very PC boot time. I didn't had that problem some weeks ago, I moved the PC to other room and used on a PC monitor for that weeks, and wasn't aware of the problem until I put it back on TV. As I said before, it works ok on a PC monitor through HDMI, also I don't have this problem with other HDMI sources. Resolution is the display native. Really weird.