[bzadel]

You probably have a very dirty volume control.  

I had a similar problem with mine. It had a lot of dirt and grease cake'd up on the contact surface.  Try removing the control completely from the chassis and pry up the three tabs that hold the control together.  The control will come apart in two pieces.  I used a Q-tip and denatured alcohol to clean the contact surface.  You'll be surprised how much crap builds up in those controls over the years. I had to repeat the process a few times until my Q-tips no longer turned black after wiping down the contact surface.

That solved my volume control problem.  I hope it works for you.

Bill