Favourite Channel List Randomly Dropped


I notice that about once a week, the box decides it will revert from using my trimmed down favourites channel list to the 'TV' list of channels (i.e. all the channels it holds), so the Guide is full of all the channels you don't normally watch, requiring regular switching back to the favourite list again.

I am aware that this can happen if, for example, you set a recording for a channel in the full TV list that isn't in your favourites list - but I'm not doing that!

Does anyone have any wisdom as to why it decides it will operate in this way, and, ideally, what if anything I can do to prevent it from switching channel lists?
Having been keeping an eye on exactly what appears to trigger this behaviour it seems, curiously, to occur when it has carried out a timer recording on ITV2!

Why would that make it happen - particularly as ITV2 is in the favourites list anyway? Bizarre...