Pixellation on DVB-S (satellite) has a number of causes, though they all relate back to one of two things – low signal levels, or poor quality signals.
If your problem is intermittant and appears to be worse in bad weather, you probably have a reception problem, caused by something interfering with the signal, or the dish alignment being out. Satellite frequencies are very high and require line of sight from the dish to the satellite; something as small as a twig in the way can cause problems so check for any trees / bushes etc that have grown up in front of your dish. Failing that your problem is likely to be that the dish alignment is out, often caused by high winds or poor installation. You can try tweaking the alignment yourself by having someone watch the signal meter on the TV screen while someone else adjusts the dish. Be wary of moving the dish more than a degree or two at a time as it’s easy to lose signal altogether.
If you have pixellating on only some channels you may be getting interference from some source. Common sources of interference are cordless phones, wireless routers, baby monitors and video / remote sender units. This is a common problem with Sky TV’s decoders and certain models of cordless phones. To determine if this is the problem, switch off the suspect source(s) and check the channels. If it’s a Sky decoder you are having problems with, Sky can solve it by either swapping your decoder for another model, or changing the LNB on the dish for one of another L.O. frequency. 2.4 GHz cordless phones are safe with the newer decoders and later model or upgraded dishes. 5.8 GHz is safe with all. If you’re having the problem with a Freeview or Free-to-air decoder, you can either replace the source of the interference with one that transmits at a different frequency, or change the LNB for one with another L.O. frequency. You may require a technician to change the LNB and set it to the correct angle.
Pixellation on select channels can also be caused by poor quality cable or connections. Cable such as RG-59 and low grades of RG-6 do not always carry the highest frequencies well between the dish and your decoder. A common culprit is the short lead between a wall socket and the decoder. Loose or poorly terminated F-type connectors (the screw on fittings on the LNB and decoder) can cause intermittant problems, as can Belling-Lee fittings (the plug in type fittings commonly used for aerials), sometimes used on a wall plate instead of the correct F-type connectors.
If none of these things solve the problem, you are best to call out a technician who can test the signal levels / quality at all junctions.