The volume control is a rheostat which can become dirty over time.
You can use TV cleaner sold by Radio Shack to help with this. You
spray it into the switch and turn the knob back and forth a few times.
The best way is to do it inside the radio (with the top off) but
you might be able to do it by removing the knob and spraying a
bit of the cleaner down the knob's shaft (depends on how long the shaft
is that connects the knob to the rheostat). A car stereo shop might
do this for you for a couple of bucks during a weekday time when
they are not busy.

Note that some Volvo radios use a digital volume control (The 816
model is like this). Don't spray cleaner in a digital control. The
digital ones have a volume control that can be spun either direction
without hitting a stop. That is, you can spin them 720 degrees (or more).