Sky Q mini buffering

[QuartzDad]

After changing channel on our mini box the channel will pause/stutter/buffer for a few seconds and continue to do it intermittently for up to 20 minutes. Problem started a few months ago after years of no problems, don't recall anything changing at the time.

Things I've changed:

TV connected to the mini box
HDMI cable to the mini box
Mini Q box
Main Q box

Still happens, any ideas?

2TB Main box
Sky FTTP broadband with Sky router
One mini Q box