[kb2fzq]

You might want to look around your area for radio frequency transmitters, such as cell towers, public service (police/fire), trucking companies, ham radio/CB antennas, anything that might transmit a high power RF signal...these signals could disrupt the TV digital data stream coming to your antenna, even if you're seeing a 93-100% signal strength, a high RF burst will disrupt the stream for just a nano-second, but enough to pixelate the picture....normally, this will happen on just a certain channel, but if the source is close enough and strong enough, it could be happening across all channels...
The pre-amp suggestion above should be your first step, if you're using one, remove it and just connect the down-lead to the antenna....at your distance to the transmitters, a pre-amp shouldn't be needed...and it may be "seeing" a stray RF signal...for example, if you can predict when the pixelation will happen, it could be a cell tower control channel searching for phones on a timed basis sending out a short, but strong signal every minute, zapping your reception for just that short burst....