I am going to purchase this TV and found a good review of the TV at --- http://www.hdtvtest.co.uk/news/sony-kdl-55hx853-46hx853-201206161860.htm
In the performance section, they mention the following which may be causing the pixelation issue ---
"Just one last note on this: during the review process, we found a bug with the KDL-55HX853 which can cause it to blur high very high frequency pixel details in motion (yes, that includes film grain and other fine textures). In fact, we detected this with our own Film Grain test pattern. We noticed that despite all the processing being shut off, the sharp pixel details in the film grain test would become hazy upon movement, although they lost none of their motion fluidity (if you’re well-versed on video processing, then you’ll understand this as motion-triggered spatial filtering). Fearing that we’d have to produce a third review in a row lambasting manufacturers for film grain blurring – something we don’t expect from a company like Sony, with their extensive post-production know-how – we were relieved to find out that it’s a bug with the [Reality Creation] “Off” switch, which can be avoided. During calibration, set [Reality Creation] to “Manual” and turn [Noise Filtering] OFF. Then, disable [Reality Creation]. If you disable [Reality Creation] without first setting the [Noise Filtering] to its lowest setting, the filter is not actually off; it’s still running in the background."
You might want to see if this helps the issue.