Technics SU-G91 volume control does not track when turned and +/- numbers/volume shown/heard.

Sorry. I haven't looked this model up, but I have your answer. I'm going to assume this unit is less than 10 to 15 years old. They no longer use, the type of volume control you think of. You can proof this out, by turned the knob left or right. It should probably have no stops, as you remember. That's because this is not a volume control, as in variable resistor. It is called a Rotary Encoder. It is basically two switches that are pushed when you turn the control to the left or right. This creates pulses(actually a high or a low) which go into the display microprocessor(one of the computers inside your unit). When the microprocessor control receiver this input, it controls another IC, called an Electronic Volume Control.

Sorry for the lengthy explanation. The issue lies with the Rotary Control. This is not an uncommon issue for any of the manufactures. This is not a DIY repair. It will require a professional Audio tech. If you need more assistance, or would like to inquire about a repair, please visit my website at audioserviceclinic.com. Thank you.