Amplifier moving spontaneously from one input to...

Question

18 year old onkyo hifi stereo amplifier will not keep a single input channel. It switches from tape deck to Tv to Dvd player etc on its own. It does not seem to be linked to heat because it will go into its dance amoung the channels whether warm (having been in use) or cold (just when we turn it on). the result is that the sound comes and goes willy nilly.