Espace radio gets louder-intermettent

I have Mark IV Espace with the standard radio in the back. For some reason it suddenly goes to FULL VOLUME. The display shows the volume increasing & I cannot stop it using the steering wheel controls. I can change the function though e.g to long wave where hopefully there is nothing playing so all I get is a very loud hiss. I cannot turn the radio off by holding the right button down either. A few seconds after it reaches full volume I can often regain control of the radio & it might not do it again for a a while or it might do it very soon afterwards. It started last winter but is now doing it in the warmer weather too. Anyone with previous experience of this issue? Could be a wiring problem or radio interference I think.