It's commonly called 'pixelization' - caused either by marginal signal strength, or a software bug in your receiver. If you are a recent sub, the latter may be the problem until your receiver catches up with all the updates. I've had Voom for six days now, and I'm just now seeing the pixelization clearing up on my receiver.
If you just notice it on SD channels it sounds like you're describing the dreaded sparklies. It seems to be chroma noise and only affects certain channels. There has been a debate as to what is causing it and we've been told they're working on fixing it.