TV changes from Virgin to Freeview on its own?

I have a TH-42PX70BA TV which has just started switchinng from Virgin Media back to freeview on its own. I switch back to HDMI1 but it changes back to freeview very soon. On freeview it keeps showing the programme bar for a few seconds and then goes off. Both hand sets have new replacement batteries and Virgin states there is no problem with their service. Can anyonr help please