It seems that the volume control potentiometer is faulty. Depending on the type you may be able to fix it but is it usually easier to replace it. This is what it may look like, there are a lot of variations. The second image shows the 'electrical' equivalent of a potentiometer. What happens is that over time either the wiper blade loses tension or the resistive material that it slides on either goes a higher resistance or depending on whether it is wire wound or carbon track it can exhibit other fault conditions.
A lot of the time the 'resistive value' of the potentiometer is printed on it and its' power rating is usually determined by its' physical size.
If you are interested I found this video about how to repair a Marantz volume control potentiometer. I do not know if this is similar to your model or not, but it may be of interest to you. it shows a 'digital' potentiometer, whereas what I have described above could be called an analogue one. Even though they work in digital circuits. It is just a different way of altering resistance values, digital being more precise. https://www.youtube.com/watch?v=y4XF9Poi...