Intermittent fault on Ruark (Vita) DAB radio?

I have a Ruark (Vita audio) R1 Mark1 digital radio which has an intermittent fault with the [alarm] [sleep] [left arrow] [right arrow] buttons. If the radio is left plugged into the mains then the buttons do not respond when pressed. However if I unplug the radio for a day, then they will start working again - but only for a limited period of time (maybe a few hours).

The radio is in perfect condition otherwise. Please help as I am very keen to try and fix this!

Thank you.
Charles