It seems to be a buffering problem, like when you download a live stream, or watching a DVD that can't track properly.
A power off reset would most likely cure it.
I find Cable TV to use a bizarre streaming setup. They seem to download content from one satellite, then re-broadcast it to another uplink, then downlink it again, for repeater coverage?
I was watching the old analogue cable box, and suddenly a Sky channel change graphic appeared, went through some channels, then selected a channel that I didn't subscribe to. SORTED! But after a few minutes, the channel was returned to the origional BAH!
So in your case OP, you might have faulty video RAM in the box itself, or the cable into your house has been damaged slightly? (water ingress)
Sky TV has these artifacts all the time, especially in bad weather. That's why I have both Sky and Cable TV. (Sky don't provide Sky3, channel 5 etc..if you don't subsribe, cable does); Cable is simply rebroadcasting the origional Sky broadcast.
If you have Cable internet, and phone, they provide a box and install it for a nominal £1 a month, if you have Sky.