I also have the HD51, I noticed that I had pixelation on a bunch of channels and noticed that it was worse when everyone in my house was using the Internet.
I tried changing the lead I made up of Wf100 cable from maplins and f-connectors from screwfix with the VM lead that goes to my router but it was still the same. This was whilst using the 5-2400mhz lab gear splitter also from screwfix which works great for a lot of people but not for me.
FYI, Using both leads, the SNR leven hovers around 78.
I read on the forum about others getting pixelation and that others had tried the konig signal booster so I bought the model (Konig Professional TV amplifier 26 dB 2 outputs active return path [KN-PROAMP32A])
This has completely resolved the issue of any pixelation on any channel :)
It could be worth buying one from Amazon, trying it out, if it works great, if it doesn't, send it back? ;)
Good luck!