Yeah , keep an eye on the quality bar and play with your dish, no fast movements and give the bar a few seconds to get an accurate reading.
1st find the channel that you have pixelation issues with, look at the frequency of that channel or channels and find the manual scan or motorized setup, then toggle thru the frequencies till you find your problem frequency and make the fine adjustments.
What channels come in crappy, this might be normal cause I've seen many channels with this problem, Amazonas has a couple of channels where the compression is such that you can compare it with grainy video or low quality digital recording.
The question is, are the channels pixelized or are you getting tiling which occurs with weak signal strength or when it rains ?