I doubt its HDMI cables that is causing an issue....unless it is only happening when you use HMDO - pop scart ithe box to the TV and see if it happens with that.
Picture braeak up can be down to a number of things...poor dish alignment, LNB starting to fail, LNB connections not good, water in the cables, nasties getting into the LNB, or last resort, the receiver is failing. Its very unusual to have it on ALL channels. You can always borrow another receiver from a freind to see if that works ok, or at least that will eliminate an issue with your receiver.?
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