There may be two problems but my guess is that there is only one cause - a very dirty volume control potentiometer.
Buy some Servisol Switch Cleaning Lubricant (Amazon) and squirt some into the control - not on the outside but onto the track of the variable resistor - you may need to dismantle.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix film projectors.