If you are using VBR encoding, make sure the maxiumum and minimum bitrates aren’t too far apart (like 7000 and 2000). Some players have difficulties switching bitrates from one extreme to another and you get stuttering or pixelizing as you described. I usually use CBR at 7000 for this reason but if I use VBR, I go 7000 and 4000 and it’s usually good with most players.