Audio Systems: abrupt volume changes, mercury sable, oil grease

Question
recently bought 1990 mercury sable.
has factory am/fm/cassette player.
audio volume changes abruptly by itself.
volume always increases from my low or medium setting to very loud. other than that, there's no pattern.  sometimes happens every 15 seconds, other times can be 20 minutes.
the knob on far left that controls volume also controls the on/off function. this doesn't work.  the unit is always on if car is on. that doesn't bother me, only the volume change does.
thanks for any help.

Answer
remove the unit out of the vehicle  open it take the volume control and spray it with a contac cleaner and wait about 1 hour and spray it with electroni oil/grease and it will work again if not i know where you can find the same unit rebuild for $50.00