Why does the input selector on my amplifier not work properly?

When I use the input selector switch on my Rotel RA-931 Amplifier (i.e. function switch for phono, CD, Tuner, Aux), the sound often breaks up on one channel, and I have to switch back and forth until it settles down. What's the reason for this, and can it be rectified? Seems a shame if the amp has reached the end of it's useful life, for something like an iffy switch.