In this case it will not be the speakers. They could not do this.  I had a look at the service manual and note that the volume control is a ganged two channel pot in the feedback loop of an IC opamp.  This is a good way of doing it, as it increases S/N as you turn down the volume.
My first suspect would be the pot.  When it does this next time see if a slight turn or bump of the volume control corrects it.  If it does the pot needs replacing.  It might also be the IC, but I would suspect the control the most.  From pictures the control looks to be totally sealed, so I doubt you can spray in cleaner, but take a look inside and see.  If you can spray in cleaner do it.