Freeview Channels Intermittently Dropping Out

[Doofus]

Screen goes black and sound cuts out for a second or two intermittently.

I think it's only affecting channels on Mux D3&4.

Can anyone shed any light, corroborate or assist?

Actions so far:

Replaced HDMI cable (3 times).
Tried different HDMI ports.
Retuned Freeview box (Humax FVP 4K GTR).